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Equipment Discussions >> Binoculars

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EdZ
Professor EdZ
*****

Reged: 02/15/02

Loc: Cumberland, R I , USA42N71.4W
"BEST OF" the Binocular Forum - Start Here
      #44730 - 01/24/04 07:51 AM

This is a READ ONLY THREAD CONTAINING LINKS TO INFORMATION for novice and expert alike embedded in binocular forum threads that are diamonds in the deep. Here you will find links to threads that all relate in some way to a primary topic. Consider this like the card catalogue in the Library that will direct you to the proper bookshelf. Comments or discussion of the subject topic should be posted back in the original threads.

ABERRATIONS AND DEFICIENCIES
Affect of Eye Pupil on Binocular Aperture
AFOV vs. TFOV and Separations for Measuring
Aperture / Magnification relationship - Exit Pupil

Best binocular overall or in a given price range
Binocular Articles over on the Reviews side of CN
Binocular Deep Magnitude Observing Charts
Binocular Mounts - Tripods - Adapters - Motions - Homemade
BINOCULAR OBSERVING REPORTS AND TARGET LISTS
Binocular Photo Gallery
Binocular Vision Summation - Two Eyes vs One Eye
Binoviewer - Scope - Binocular Equivalents
BOOKS for Binocular Observers

Chromatic Aberration, Triplets, Semi-Apo, Apo
Cleaning Lenses - Binoc Repair Eyeguards - Caps
Coatings
Contrast

Dark Adaptation, Eye Pupils, Lights
Depth of Field and 3D or not 3D!

Edge Performance - Sharpness of Field
Exit Pupils 5mm vs 7mm

Favorite Binoculars in Various Sizes
First Binocular Purchase? Questions?
Fixed Focus Binoculars

GOOD STUFF FOR BEGINNERS
Handheld Binocular Observing
How to Focus Binoculars

If They Look the Same, What Could be Different?
In Pursuit of Cheap Binoculars. What to Expect.

Limiting Magnitude and NELM

MEASURING / TESTING Optical Aspects of Binoculars

Need Binocular COLLIMATING or Repair?

Philosophy and Binocular Ownership by Bill Cook
Polish Binocular Tests and Reviews
POLLS in Binocular Forum - take a poll
PRISMS - BaK 4 vs BK 7 - Roof vs Porro

Resolution and Visual Acuity
Resolution Testing and USAF Res Charts

Surface Brightness of Extended Objects

Telescope, Binocular
Terrestrial Viewing
Testing Binoculars in the Store Before Buying
Twilight Factor - Visibility - Binoc Index

Vintage and Classic Binoculars Discussions
Vintage and Classic Binoculars Discussion (Part 2)

What can be Seen in Various Sizes Binoculars
Writing a Binocular Review

Zoom Binocular Cautions!


ATTENTION: This post is BEST VIEWED IN THREADED MODE (the word FLAT must appear on the upper right flat/threaded button) so you can see the titles of each "Index Card" subject. Once you find your topic enter that post and you will find links to posts related to that subject. There are now over 350 links grouped into about 40 topic index cards. You will waste a lot of time and effort scrolling if you don't first set to threaded mode.


Thanks and Enjoy,
Moderator, edz
Questioning one’s own understanding is probably the hardest question of all.

Edited by Erix (10/24/12 12:16 PM)


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EdZ
Professor EdZ
*****

Reged: 02/15/02

Loc: Cumberland, R I , USA42N71.4W
Twilight Factor - Visibility - Binoc Index new [Re: EdZ]
      #44857 - 01/24/04 01:43 PM

What is Twilight Factor?
When looking at specs for binoculars, I see a line item called "Twilight Factor" and it has a number like 16, 18.3, 20.5, etc...
What is this number telling me?

Multiply the magnification by the objective size in millimetres (for example a 12 x 50 = 600)

Then calculate the SQUARE ROOT of this number ( in this case it would be 24.5 )

The HIGHER the Twighlight Factor , in theory at least the BETTER the binocular will perform in POOR or FADING light.

So whilst exit -pupil (arrived at by dividing objective size by magnification --in this example about 4.2) provides increased BRIGHTNESS as it increases, there comes a stage in lighting conditions where increased MAGNIFICATION helps you see things better in darker situations.

Both of these factors, twilight factor and relative brightness are very important --but do not tell the whole story.

ACTUAL brightness and TWILIGHT PERFORMANCE are also affected very much by QUALITY of glass and coatings and optical designs incorporated ( e.g prism types )

So a top quality $1000 10x40 for example may very likely in reality outperform a $100 12x50 in terms of TWILIGHT PERFORMANCE , even though the math would lead you to believe otherwise .

TWILIGHT PERFORMANCE is in my opinion a very important and often under-rated and overlooked factor with binoculars.

Hope this helps - regards -Kenny.

The original thread is at
What is Twilight Factor?


What Is Visibility Index?
Various noted individuals over the years have adopted scales that attempt to represent relative performance of binoculars, based on some ratio of magnification and aperture. Roy Bishop created the Visibility index that is Mag x Aperture. Alan Adler created the Astro Index that is Mag x Sqrt Aperture. EdZ created an index call the Binocular Performance Index (BPI) see CN Reports - Binoculars - How to Understand Binocular Performance (Feb. 2003) that adapted a scale based on the Adler Index (mag x sqrt ap), but then applied factors based on quality (coatings, contrast, polish, light cutoff, etc.). While I have always been a stong supporter of magnifiaction having greatest influence, that can sometimes be very misleading. In my opinion, it is of far greater importance to consider the intended targets and then determine on which (mag or aper) you should place greater emphasis.
However, the brief explanation of how the indices are determined is this:
(For purposes of this discussion, I am assuming all instruments of equal quality. If so, my BPI would give the same comparative result as Adler Index.


Roy Bishop Visiblity Index
20x60 = 1200
15x80 = 1200

Alan Adler Astro Index
20xsqrt60 = 155
15xsqrt80 = 135


this thread discusses the difference between Roy Bishop's Visibility Index and Alan Adler's Binocular Index.
Is aperture king? Does aperture rule?

further discussion on the topic of Indexing
What does the 'Visibility Index' tell you?
this has explanations of when these indices apply and when they do not.

Quote:

this Sky and Tel binocular article ...makes a brief mention of Mr. Bishops article I mentioned. The Observer's Handbook article is much more indepth but contained in the first link is the general idea of his "visibility factor". The second link is his enlarged visibility factor chart from the Sky and Tel article:


http://www2.wwnorton.com/college/astronomy/astro21/sandt/powerbinocs.html


http://www2.wwnorton.com/college/astronomy/astro21/sandt/images/pabin2.gif

Stan





Also see this explanation of Binocular Performance Indexing - BPI
this thread explains it in brief. Following in the footsteps of Bishop and Adler, I developed
Binocular Performance Index, BPI, which is Adler index adjusted for quality.

Alan Adler's article referencing his index can be found here
http://www.weatherman.com/binadler.htm


What the BPI attempts to do is rank binoculars dependant on their performance characteristics. A normal binocular gets no adjustment. The binoculars that exhibit the best attributes, contrast, transmission, lack of aberration, get adjusted up. Binoculars that exhibit some aberrations or are not well corrected for contrast and transmission, get adjusted down.

Premium coatings and full exit pupil light distribution contribute to greater total light transmission. Well corrected lenses, high polished finish, premium baffels and premium coatings and premium eyepieces contribute to lower scatter and high contrast and better resolution. Transmission, Contrast and Resolution (these are related in various ways) are considered attributes that will have an overall additive or deductive contribution to performance. I have arbitrarily assumed to use a factor for two of these that would either contribute or deduct 10% from the overall index. I used Adler Index (mag x sqrt aperture) as the base.

I would rank the BPI of two different binoculars as such:

Nikon SE 12x50
premium coatings, premium baffles, low scatter, high contrast
lack of aberrations, high resolution, high transmission
Bishop = 12 x 50 = 600
Adler = 12 x sqrt 50 = 85
EdZ BPI = 85 +10% for contrast +10% for transmission = 103

Barska 15x70 - Celestron 15x70 - 2002(1st) version Oberwerk 15x70
less than normal coatings, more scatter, lower contrast,
higher than normal aberrations, lower resolution, lower transmission
Bishop = 15 x 70 = 1050
Adler = 15 x sqrt 70 = 125
EdZ BPI = 125 -(minus)10% for contrast -10% for transmission = 100




this article explains it in detail
This study includes explanations of the influence of magnification vs aperture as relates to point sources, (stars in clusters)
How To Understanding Binocular Performance Indexing - a CN Lab Report by EdZ
The testing and methodology that led to develoment of the BPI


Edited by EdZ (02/13/10 09:49 AM)


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EdZ
Professor EdZ
*****

Reged: 02/15/02

Loc: Cumberland, R I , USA42N71.4W
AFOV vs. TFOV and Separations for Measuring new [Re: EdZ]
      #44859 - 01/24/04 01:48 PM

The True field of view of the binocular is equal to the Apparent field of view of the eyepiece divided by the magnification. Tfov = Afov / mag

If the Tfov is given as feet at 1000 yds., then Tfov in degrees is (Tfov in feet at 1000yds) / (1000yds x 3 feet per yd) = radians. Then radians x 57.3° per radian = Tfov in degrees.

If the Tfov is given as degrees then the Tfov in feet at 1000 is (Tangent Fov degrees) x 3000 = Fov in feet at 1000 yds.

Binocular Tfov Conversion Tables
given the measure usually printed on your binoc, i.e. 51M at 1000M, look up your field of view here.

But not all binoculars will actually measure what is specified or what is printed on the binocular. In fact many do not. So if you want to know the actual real field of view, measure it in the field by observing a star field.

A useful list for Binocular Observer's is this one with many star fields measured so you can determine the actual True Field of View of your binoculars
How To Measure TFOV Of Binoculars


A more recent discussion on the topic
Apparent FOV, True FOV and Distortion



Nice collection of constellation charts with separations printed on the charts is here
Stellar Separations for Determining Field of View
Chart of Sagitarius
Chart of Leo
Chart of Lyra


Southern Stellar Separations for Determining Field
Chart of Ara
Chart of Musca
Chart of Crux
Chart of Triangulum Australe


Bino FOV Circle templates for a wide selection of charts


Wide-angle binoculars can have narrow Afov eyepieces and Narrow angle binoculars can have wide Afov eyepieces. Except at the narrowest and widest extremes of True field of view, there is no standard rule you can follow.

Some binoculars do not actually measure in the field what is stated for Tfov. Usually the higher cost premium binoculars do measure as stated.

Half of all binoculars I measured have Afov eyepieces between 60° and 65°. They cross a range from narrow to wide True field of view. There are some but few wider eyepieces in use.

What may seem like a narrow 4° field of view at one power would be wide for a 15x or 16x binocular. A 3.5° field of view would be wide for a 20x binocular. On the other hand, a True field of view less than 3.0° seems narrow for ANY binocular.

Some wide Afov eyepieces have excellent performance. Fujinon 16x70 and Oberwerk 15x70 use 64° Afov eyepieces and are sharp to 80%+ Tfov. In comparison, Orion Giant 16x80 uses a much narrower 53° Afov eyepiece and its sharpness characteristics are poor in the outer 40% Tfov.

Some 8x binoculars with a 6.5° field of view may be considered the low end of a wide True field of view for that power binocular. This Tfov is produced with a 52-53° Afov eyepiece, not considered a wide Afov eyepiece. They are sharp to 80% of the True field of view.

Most of the 22x and 25x binoculars on the market are advertised as 3.0° Tfov, but are measuring less than stated. If any 22x or 25x binoculars measure over 3.0° Tfov, they are using eyepieces with Afov approx. 66° to 75°.

You will find that your eyes are not capable of seeing the edges of the filed in eyepieces with an apparent field of view greater than 65° to 70°. Some binoculars advertise 'eyepieces with 80° apparent filed of view.' Well the eye simply cannot use all that at once. The eye has limitations and generally for most people that falls in a range between 60° and 70°.

edz


Apparent Field of View vs. True Field of View
Original thread Confusion with Field of view Differences


Find more info at

Original Thread Apparent FOV vs True FOV


A lengthy explanation of True field of view and Apparent field of view resides in this thread. A selection of the main points pasted here:

Q
I've done a search on FOV and still don't really understand the differences between real TFOV and apparent AFOV.

As a more specific example, Pentax have two roof prism binoculars with specs as follows:
Pentax DCF SP 8x42 (TFOV 6.3 degrees, AFOV 50.4 degrees)
Pentax DCF SP 10x42 (TFOV 6.0 degrees, AFOV 60 degrees)

How does the 10x42 which has a narrower True FOV end up with an Apparent FOV greater than the 8x42? Does this mean a wider visual view through the binoculars (i.e. more sky)?

A
The narrower TFOV means it will see less sky. The apparent AFOV relates to the width of the image generated by the eyepieces. Dividing that by the magnification of the complete instrument yields the true TFOV in terms of actual degrees of arc of sky that will be displayed.


A
In your example the Pentax 8 x 42 has (TFOV)R.F.O.V. of 6.3 degrees

This is more usually described as TRUE field of view(TFOV)

What does 6.3 degrees actually mean ?

One degree can also be expressed as 52.4 feet / 1000 yards
Or as 17.4 metres / 1000 metres

What this means is that if you looked through a binocular with a ( very narrow )one degree TFOV , then at a distance of exactly 1000 yards from where you were standing you would only be able to see a stretch of land or object that was 52.4 feet wide . Anything wider than that would be out of the area which you can see through the binocular.

Or , to convert to metric measures , if you looked through the same binocular , then at a distance of 1000 metres from where you were standing , you would only be able to see a stretch of land or object that measured 17.4 metres wide.

So with a 6.3 degree TFOV , to get the equivalent figure in feet per 1000 yards or in metres per kilometre , you would multiply the figures quoted above by 6.3.

So at 1000 yards you would be able to see a stretch expanding 6.3 x 52.4 which = around 330 feet at 100 yards

In metric , 6.3 x 17.4 = around 109m per 1000m.

The Apparant Field of View simply means the TFOV as described above MULTIPLIED by the MAGNIFICATION.

So a 10 x 50 with a 5 degree TFOV provides a 50 degree AFOV

In this case , knowing the requirement is for a 5 degree TFOV with a 10 x power factor , the lens designer selects what is universally accepted as a 50 degree eyepiece of focal length sufficient to provide 10x magnification within that binocular system.

Thus is why telescope eyepieces are always described by their APPARENT field of view rather than by their TRUE field of view.

For example a plossl e.p usually offers around 50 - 52 degrees AFOV whereas a Televue Nagler can offer as wide as 82 degrees AFOV .

The True fields of views offered by such e.ps in a telescope depends on the focal length of the telescope which in turns affects magnification.

The greater the magnification the narrower the TFOV and the lesser the magnification the wider the TFOV.

A TFOV that would be considered annoyingly narrow for say a 10x binocular( for example 4 degrees TFOV )only really appears so because the AFOV would be only 40 degrees ,arrived at by multiplying 10 x 4 .

However if you had say a 16x binocular with a TFOV of "only" 4 degrees -- the AFOV would be 64 degrees(16 x 4), and as such would appear to be perfectly acceptable because the extra magnification is making up for the narrow field.

What is an "acceptable" AFOV depends very much on individual choice and experience. One can and often does "get used to" a particular AFOV.

Kenny


Field Of View
a discussion inthe eyepiece forum regarding the ability of the eye to scan around the Afov of various eyepieces from 30°Afov to 100°Afov. Also discussed is the range of the eye's ability to see resolution, see clearly, peripheral vision and extent of peripheral movement.



Edited by EdZ (06/10/10 01:28 PM)


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EdZ
Professor EdZ
*****

Reged: 02/15/02

Loc: Cumberland, R I , USA42N71.4W
Exit Pupils 5mm vs 7mm new [Re: EdZ]
      #45203 - 01/25/04 07:50 AM

Also read
Affect of Eye Pupil on Binocular Aperture

Exit Pupil is the beam of light exiting out thru the eyepieces. The diameter of that beam can be found by dividing objective diameter size by magnification. Larger Exit Pupil provides increased BRIGHTNESS. But there are some very important points for consideration.

exit pupil 7mm vs 5mm
If you have a 7mm eye pupil then you get all the light from say a 7x50. But if your eye pupil does not get that large, and many don't, then the effective aparture that you are getting out of the binocular is reduced. So, a person with a 6mm eye pupil, when using a 7x50, it actually only getting 7x42. A person with a 5mm eye pupil is getting effectively a 7x35.

7mm exit puplis will provide a much brighter image than a 5mm exit pupil, BUT ONLY IF you have very very dark skies. Then you would have good contrast. But if you have any amount of light pollution, let's say you have mag 5 skies or worse yet mag 4 skies, then that large 7mm exit pupil may work against you. The brighter sky background will be brighter in the binocular with a 7mm exit pupil and it will tend to wash out the contrast for all objects. In this case a 5mm exit pupil might be better.

I would seldom recommend a 7x50 binocular to anyone as it is a specialty binocular far more affected by limitations of exit pupil and sky conditions and in most instances will not perform as it was intended.

I would never recommend a 7mm exit pupil 7x50 as a first astronomy binocular.

edz



Several threads have embedded discussions on exit pupil. Following are discussions and links to the original threads.


One question that a beginner often asks about is exit pupils. There is so much talk about exit pupils that it can be thoroughly mind-boggling. This primer will set you on the right path. When you begin to yearn for more information visit the Best Of link dedicated to exit pupil.
Binocular Primer - An explanation of Exit Pupil, Eye relief and Field of View



Another member asked "I think these (7x50 celestron ultima) are great binoculars...BUT... I find they amplify sky glow or produce it? ... Is this because of the rather large exit pupil(7.1mm)? I don't see this glow in my 15x70's and I'm guessing that it's because of the 4.4mm exit pupil or the higher magnification."
Exit Pupil Affects the Performace of Your Binocular - Make the Right Choice




This discussion took place here several years ago.
Q
Is the difference between a 5mm and 7mm exit pupil really that significant (I've been looking at the Pentax 7x50 PCF WP's)? The 7x50's give a much wider FOV (324ft/1000yds vs. 261ft/1000yds), which I would think is more important for bino star gazing...Am I missing something fundamental here?

A
Generations of binocular designers who have forgotten more about optics than I will ever know can surely not all be wrong or stupid to have designed so many 7 x 50 , 8 x 56 , 9 x 63 , 10 x 70 and 11 x 80 binoculars over so many years.

"Accepted theory" seems to indicate that, generally speaking, people over around 50 years of age have maximum pupil dilation of only around 5mm in darkness. I bravely pronounce that this assumption is somewhat flawed if not complete poppycock. A noted regular contributor to this wonderful site, let alone forum , who is over the age of 50 , has recently had his dilated pupils measured at 7mm --and that not in COMPLETELY dark -adapted circumstances .

There may well be very good arguments that a binocular with an exit pupil as large as 7mm is performing nowhere near it's maximum potential , but that is not to say that the same binocular will not provide a much more "bright, easy and comfortable" image than would the same (aperture) with higher power eyepieces.

Because terms such as "brightness" "ease of view" and "comfort" have no measurable units, they are unlikely to stand-up well in any technical tug of war against the advantages of higher power (and thus smaller exit pupils) that CAN be measured in the very significant and universally accepted and very real properties of contrast and resolution.

So it is the old argument of "swings and roundabouts" , another example of the fact that no matter how good any particular binocular is for any specific intended purpose, there is always a compromise.

When you gain in power you lose in brightness and field of view, when you gain wider fields of view you tend to lose "flatness" -- a desired quality for stargazing.

Another "con" of a large exit pupil is that it amplifies any human eye astigmatism , making it virtually impossible for anyone with such afflictions and NOT wearing glasses or lenses to correct it, to attain "crisp ,sharp, clear" images , which again are terms not to be found in an optics expert's vocabulary but which are more "artistic" terms ... intended to convey just about EVERYTHING desirable about a magnified image.

I happen to prefer very wide fields of view with hand-holdable low power and "easy to see with " 6mm exit pupils, which is why I wouldn't swap my Zeiss 7 x 42 for any binocular in that size or price range.

All this said , I would estimate that 90% of the time a 5mm exit pupil is ENOUGH IF NOT TOO MUCH for binocular astronomy pleasure and performance.

Regards -Kenny.


Tom,
First, if you do not have eye pupils that dilate beyond 5mm, then 7mm exit pupil does you no good. [With the Exception that Larger exit pupils do make it much easier for your eyes to acquire and hold the image]. But if you do have eye pupils that dilate beyond 5mm, then there are applications where the 7mm exit pupil will show greater performance. That is primarily on broad diffuse extended objects. But only if you can get to dark enough skies.

What do you give up to get 7mm exit pupils? Image size. But if the image is broad and diffuse, do you need a larger image scale? Maybe not.

The binocular that would be best for broad diffuse extended objects is not the same binocular that would be best for most other objects. For most everything else, a higher magnification (that produces a smaller exit pupil) provides a larger image and a bit deeper limiting magnitude that allows seeing more. More stars in wide-open clusters, more resolved stars in dense clusters, a little bit more definition in doubles.

All that is probably why many of us own more than one binocular. Kenny even has his terrestrial favorites, I my backpacking favorites.

It's not all in the exit pupil either. A higher contrast, sharper image from a smaller exit pupil can exceed an image from a larger exit pupil if not from an equally high quality binocular.

My 16x70s exceed my 15x70s and my 10x70s.
My 12x50s exceed my 10x50s.

But in either case not in field of view. However, fov can lose some of its relative importance if the image is considerably better.

edz


More Info can be found at:
Original Discussion Embedded in Thread "Nikon 10 X 50 Cf Action Vi Binoculars"


EXIT PUPILS
Q
Are large aperture, low mag binoculars pointless?

It seems that large aperture, low mag. binoculars are wasteful. What is the reason for their creation if the extra aperture serves virtually no purpose?
(Would I be better off using my brother's 15x45 binos... or at least be just as well served by a pair of 11x50's because the other 20mm of aperture are wasted?)... Please explain this to me...

A
Binoculars that provide very large exit pupils are designed with maximum brightness in mind. That is, they will deliver the maximum bright image of faint diffuse objects. This is a benefit when viewing faint diffuse nebula or large comet tails or the extended arms of galaxies like M31, M33 and M101, things of that nature.

For almost all objects other than faint diffuse objects, magnification provides greater benefit. Magnification allows you to see deeper in magnitude, putting more stars in the field. This is a benefit in open clusters and star fields like the M24 star cloud or even scanning the Milky Way.

Magnification also provides larger image scale that allows you to see objects that otherwise would have been too small for the eyes to perceive.

Remember, a larger objective gathers more light, allowing you to see faint extended objects. A larger exit pupil delivers a brighter image to the eye, again helpful for faint extended objects. Magnification is what allows you to see most small detail.

Whether large aperture low power binoculars are the right choice in binoculars depends very much on what you intend to use them for.

edz

More Info can be found at:
Embeded in the Original Thread "Are large aperture, low mag binoculars pointless"


A good point is made here;
There would appear to be a popular misconception , even amongst experienced binocular users , that an exit pupil from a binocular that is greater than the dilation of the human eye -pupil , is completely "wasted".

There are two very good reasons why a binocular benefits from having exit -pupils greater than diameter of the eye pupil.

1.Avoids the necessity for setting the IPD perfectly, and then the need to perfectly center your eyes to the exit pupils.

2. Permits your eyes to swivel in their sockets, without losing the image, as the iris of your eye slides off to one side as you look toward the edge of the field of the binoculars.

However
For a 10x70 binocular with a 7mm exit pupil, if your maximum dilated eye pupil is 6mm, then light gathering thru the binocular will be determined as if deleivered by a 10x6mm eye pupil = 60mm effective aperture.... Resolution will be determined as if delivered by 70mm x (6mmeye pupil/7mm exit pupil) = 60mm lens, same as light gathering. In general, if your eye pupil does not dilate as wide as the exit pupil of the binocular, your eye effectively reduces the aperture of the binocular.

More info can be found at
Original discussion in thread "Relation: aperture-mag when view extended objects"


Edited by EdZ (02/13/10 09:47 AM)


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EdZ
Professor EdZ
*****

Reged: 02/15/02

Loc: Cumberland, R I , USA42N71.4W
Zoom Binocular Cautions! new [Re: EdZ]
      #45514 - 01/25/04 08:58 PM

Before you decide to go ahead and purchase a ZOOM binocular, please read this bit of advice:
Noted Repair Shop Caution on Zoom Binoculars

Then you can read these various posts by some owners

Zoom binoculars (at a reasonable power/aperture ratio) would be a wonderful thing to use if some manufacturer could overcome the basic design flaws of zooms: narrow FOV, dim images, and collimation problems.

My Nikon 8-16X40 XL Zoom is the best zoom I've seen, very sharp optics almost to the edge of the 5.2* FOV (wide for a zoom). However, when comparing it back to back with other good quality binoculars at 8X, 10X, and 15X, the XL Zoom is somewhat dimmer. This may be in part due to the smaller exit pupil (e.g., 2.6mm at 15X vs. 4.6mm for a 15X70), but probably mainly due to inherent light loss of the zoom lens since they are also dimmer, though less so, at 8X and 10X.

Despite this flaw, having the ability to "dial up" the right magnification for the object you want to view is a very nice feature. I'm not sure if aspherical lens technology, better coatings, etc. could overcome the inherent design flaws of zoom bins, but if they could be designed as good or even nearly as good as fixed power bins, they would surely be the most popular bins on the market.

Until then, the Nikon XL Zoom is probably the best compromise possible. The new dual power Leica Duovids look very promising, but are very pricey and heavy for handheld use. If anyone has used one of them (8+12X42) or 10+15X50), please post a review. Thanks.

Brock

See the entire discussion at
100x zoom bino's


I have a Nikon 8-16X40 XL Zoom, and it's great for astronomy, particularly from a light polluted site where smaller exit pupils are needed to increase contrast. It's also good for birding/wildlife observation (close focus 15'). Unlike many zooms it has a fairly wide true FOV (5.2*), which translates to a 83* AFOV at 16X. Of course, the exit pupil at 16X is 2.5, so the views are a little dim at the extreme. The best views are between 10 - 14X. The XL Zooms have very sharp multicoated optics, a bit sharper than my nikon 10X35E2, but with a flatter field, and sharp almost to the very edge.

The XL Zoom weighs 30 oz., less than a 10X50 Ultraview

The XL Zooms are discontinued, but a few companies (hotbuys, for example) still sell them for just under $600.

See the entire discussion at
Anybody using zoom binos?


Check out the previous thread on zooms "Anybody using zoom binos?" started by Tom Trusock. I posted comments about my Nikon 8-16X40 XL Zooms.

I've seen other zooms, and didn't like them, always cheap, dim, small FOV, and not sharp. The XLs are the exception to the rule, however, they DO suffer from miscolimation at close focus.

The alternative type of multiple power bin is the Duovid (8 + 12X42) and (10+15X50). They have two powers instead of a whole range. I haven't heard any rave reviews about them yet, but they're fairly new and VERY expensive so not many people probably have them. However, from reading about them, I think they represent a better alternative to zooms.

See the entire discussion at
Barska 12-60 X 70mm Zoom Binos

See also this thread
Zoom Binoculars for less than $150

Note these comments by various posters. The thread link follows.

note that zoom binoculars usually have a lower FOV then fixed magnifactions. They are however very helpful if these will be your main viewing tool.

Generally, zoom binoculars, although they provide a neat feature, are considered inferior to fixed mag binocs. One thing is for sure, if they go out of allignment, they are nearly impossible to get back in, even by the guys in the shops.

I've not tried the 10 -30 x 60s but although the specs may sound quite reasonable there are always severe limitations with field of view with all zooms I've tried.

See the entire discussion at this thread
Yeah, size matters... but what ELSE does?

Barska 12-36X70 zooms (mini-equipment report)


Edited by EdZ (02/13/10 09:46 AM)


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EdZ
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Coatings new [Re: EdZ]
      #45530 - 01/25/04 09:31 PM

PHASE COATING
Phase coating is special coating and is added to the prisms ONLY in roof prism binoculars to PREVENT destructive interference, actually referred to as constructive interference. Since roof prisms may have on surface that is below th angle of total interanl reflection they have one surface covered with a reflective mirror like material. Somewhere along the light path in the prism, the light has the potential to get out-of-phase. Phase coatings are applied to correct this problem. This is intended to act differently than anti-reflection coatings and is intended to improve the performance of the roof prisms. Standard porro prism binoculars do not use phase coating.
Links to articles on Phase Coating


See this thread discussion where Holger Mmerlitz explains phase correction and provides links for further study
Resolution and Phase Correction

All the remaining discussion here will refer to Antireflection (AR) coatings.


ANTIREFLECTION COATING

Coatings are added to porro prisms and lenses to improve the light transmission of the glass. Also they add destructive interference to cancel out the light reflected off the surfaces. This helps eliminate reflectance and ghost images, a desirable result in lenses and roof prisms, improving contrast.

An uncoated lens surface can reflect 4% of the light hitting it. Magnesium Flouride (MgF) single coating can reduce that reflected light to 1.5%. Properly applied multicoatings can reduce that reflectance to 0.5% or 0.25% per lens surface. Improperly applied multicoatings may do no better than, or not even as good as, good single coatings.

A binocular may have as many as 14 lens surfaces. With the exception of extremely high level of performace of some superior coatings, even a properly muticoated binocular can transmit at best only about 93% to 96% of incoming light. The same binocular with all 14 surfaces only single coated with MgF may transmit only 79% of the light.

Coatings are a very important product in the transmission of light. In this age of products, no one should purchase binoculars that have uncoated surfaces.


Color provides no clear indication of the quality of coatings. There are a variety of chemicals used to develop multi-coatings. Pentax SMC coatings appear purple or rose. Orion Ultraview coatings appear blue/green/purple, depending on what angle you look at them. Some Nikon coatings have a yellowish tinge to them. Single layer MgF coatings appear blue and if applied properly can be better than improperly (too thick) applied multi-coatings. Coatings that are applied too thick can appear bright green.

See this discussion for an explanation of the colors you see reflected from the coatings.
Prisms and Coatings - Differences

Generally, plain blue coatings is an indication of a single coated MgF surface. While MgF coatings are not bad, and in some instances may be better than multicoated (poorly applied) surfaces (ie. compared to the very green coatings mentioned below), single coated surfaces do reflect more light, reduce transmission and may cause internal reflections. I see more internal reflections (ghost images) in my single coated MgF binoculars than any other type of multi-coated binocular.

Coatings that have a multi-hue, blue-green, green-purple, blue-purple, purple-rose, purple-amber, can all be indications of properly applied coating. In this case, a comparison of reflections off the coatings may give some indication of reflected light and hence transmitted light. Generally, these are what I see on better binoculars.

Green coatings, especially very green coatings, may be an indication of a misapplication of the coatings during the process. Phil Harrington has been giving this advice for well over a decade. In order for coatings to work properly, they must be a precise thickness. The precision of the thickness of the coatings is measured in nanometers. Coatings that are applied too thick often end up appearing bright green in color. While the coating surface itself may appear to reflect very little light, the coatings are not performing as antireflection coatings were intended to perform. They may in fact be canceling out portions or certain wavelengths of light, allowing less light to get thru.

It is not uncommon at all to compare binoculars and find a pair with bright green coatings that simply cannot see as much light as another pair, which for all intent and purposes should produce the same result, but by virtue of better coatings can see several tenths of a magnitude deeper. Just to get a better understanding of the importance of the small fractions of magnitudes, a gain of 0.75 magnitude is approximately a 100% increase in total light.

Web Article on Coating Colors


A better indicator than color of the quality of antireflection coatings may be any reflections you can see when looking into them. It is difficult to see any reflections at all in Fujinon coatings. Look into the objectives of an Oberwerk BT100 binocular telescope and you will barely see the outline of your head. You will not see any detail in the reflected image of your face. Look into a binocular with lesser quality coatings and you may be able to see your full reflection in color with detail in your face. Any light you can see reflected coming back off the lens is not passing thru the lens. Premium AR coatings should minimize this reflected light.

You can also shine a flashlight into the objective lens of your binocular and you will see 6 or 8 reflections of the flashlight. Although this will not indicate the quality of the coatings, expect at least to see all colored reflections. White reflections may indicate uncoated glass surfaces. (Note - a cemented doublet objective may produce a white reflection as the two surfaces that are cemented together act as continuous glass and need not be coated). In this age of astronomy, no one should be observing with uncoated lenses.

Here's a simple test to compare Coatings from one model to another
Look for reflections in the coatings

Importance and value of coatings

Nikon 12x50SE coatings vs. Fujinon 10x70 FMT-SX

Coatings compared on a dozen different binoculars

Coatings differences on different models of Pentax binoculars



Q
I would make sure (binocular)are fully multicoated (oberwerk calls them broadband coatings: what ... is that!?).
A
As regards "Broadband Coatings" I asked the same question to Kevin at BigBinoculars.com recently and here was his reply :

Astromart Forums: Astro Binoculars
Re: Broadband coatings
Posted by Kevin Busarow on 4/8/2003 3:43:23 PM

Same question came up in the Binocular Astronomy Yahoo group a while back, and here's what I wrote-
======================================================
Broadband is a multilayer coating (5-7 layers) that has minimal reflectivity (.2% to .5%) across a very wide (broad) range (band) of the spectrum (450nm to 750nm). It's simply the best anti-reflection multi-coating the Chinese manufacturers can do with the equipment they have.

Common MgF2 coatings average 1.3% to 1.5% reflectivity across the same range. While the difference in efficiency with better multi-coatings is small, because binoculars have many air-to-glass surfaces, broadband on every surface (fully broadband multi-coated) can significantly improve overall light transmission. There is also a degree of improvement in contrast, as less light is scattered at each surface. You won't find this level of multicoating on the least expensive Chinese binoculars simply because of the time it takes to do the multiple layers. Some Oberwerk models have this level of multicoating now,
and all models will have it as new stock is received.
Kevin Busarow

Read the Manufacturers Explanation of levels of Coatings


see the embedded discussion at this thread
Anybody using zoom binos


While smaller exit pupils will certainly increase contrast, and as an added bonus compensate for eye aberrations, coatings are very important too. Although the 8X32 SE has a larger exit pupil than the 10X35 E2 (4mm vs. 3.5mm), the "superior" coatings help admit more light and hence help provide better contrast. Similarly, the contrast in my 15X70 '03 Oberwerk is better than my 20X80 Burgess though the exit pupil is larger (4.6mm vs. 4mm), because the Obies have better coatings.

See this discussion at the following thread
Performance of 8.5x44 vs 9x63 vs 10x50?

Q
I bought the Oberwerk 15x70 this year. Is their any obvious visual differences between the 2002 and 2003 models? I would like to know for certain if mine are the 2003 model.
A
The most obvious outwardly noticeable difference is in the coatings. The '02 model has only blue coatings on the eyepieces. The '03 model has the same coatings on the eyepieces as it does on the objectives and it gives off a green/purple color.

A
There is a significant difference in the coatings between the Oberwerk 2002 and 2003 models. While I had noted the 02 model has the most reflections off the lenses of various binoculars I was testing at the time, the new 03 model has significantly less reflections off the objective lenses. At the eye lens, the 02 models appears to be single MgF coated and the 03 model seems to have the same multicoating as the objective.

The overall view thru the new 03 model seems to be noticably brighter than the older 02 model. Viewing around the area of the Cygnus Milky Way, there was more and brighter faint star background visible in the new 03 model.

See the entire thread at
Oberwerk 15x70 2002 vs 2003


I check the coatings in my binoculars on the Moon by looking for reflections inside. If there are reflections, then the coatings arn't so great.
See this post for why you may not be able to tell very much about the coatings by Observing internal reflections of the moon


Edited by EdZ (02/13/10 09:45 AM)


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Aperture / Magnification relationship - Exit Pupil new [Re: EdZ]
      #49488 - 02/03/04 09:04 PM

Aperture / Magnification relationship in binocs

Most recently, this discussion took place touching on all aspects of the above mentioned topic. This thread may be best read in it's entirety.

Comment about Exit Pupil Size


Numerous other threads have discussion related to the same topics.

Are large aperture, low mag binoculars pointless?

See the following comments from some of our forum participants:

It seems that large aperture, low mag. binoculars are wasteful. What is the reason for their creation if the extra aperture serves virtually no purpose?
(Would I be better off using my brother's 15x45 binos... or at least be just as well served by a pair of 11x50's because the other 20mm of aperture are wasted?)... Please explain this to me...

I’ve have come to the conclusion that with binoculars, like everything else in life, it all boils down to compromise. From my …experience I’d have to say that large aperture/low mag binocs are most definitely NOT pointless, they just fill another niche. ... it depends on what your going to use them for.

whilst neither low mag nor large aperture binoculars are remotely "pointless" both categories DEFINITELY have their place in the pleasures of astronomy, unless you are viewing from an exceptionally desirable, non light polluted site, then the combination of LARGE (70mm +) AND LOW power (<12x) would seem to make little sense to me.

when you increase the magnification the image gets a little dimmer so if you are looking at some of the larger nebulae in dark skies the 11x is probably more appropriate.

See the complete comments at this location
Are large aperture, low mag binoculars pointless?!


Masking a Binocular Aperture
Does aperture rule in bino land?
masking is one way to see the effects of aperture vs magnification. This post has a discussion related to some of those effects.
Also refer to the article
How to Understand Binocular Performance


Binoculars that provide very large exit pupils (large aperture-low mag) are designed with maximum brightness in mind. That is, they will deliver the maximum bright image of faint diffuse objects. This is a benefit when viewing faint diffuse nebula or large comet tails or the extended arms of galaxies like M31, M33 and M101, things of that nature.

For almost all objects other than faint diffuse objects, magnification provides greater benefit. It allows you to see deeper in magnitude, putting more stars in the field. This is a benefit in open clusters and star fields like the M24 star cloud or even scanning the Milky Way.

Magnification also provides larger image scale that allows you to see objects that otherwise would have been to small for the eyes to perceive.

Most people will argue that the size of the objective lens is what determines the resolution, and yes this is true. But how much of that resolution the eye can see is dependant on how large the resolved image is magnified. Just about every binocular objective size provides more than enough resolution for the eyes. Without sufficient magnification, the eye will not see what has been resolved.

Take an example of an observer with acuity that allows seeing objects as separated when they are magnified to an apparent size of 150 arcseconds. When viewing an object like M36 using 15x binocs, stars that are as close to each other as 10 arcsec will be seen as separate objects. The same observer using 11x binocs will not be able to see stars only 10 arcsec apart as separate objects, they will appear as one. Hence, 11x will not resolve as many stars in M36.

Remember, a larger objective gathers more light, allowing you to see faint extended objects. A larger exit pupil delivers a brighter image to the eye, again helpful for faint extended objects. Magnification is what allows you to see most small detail.

Also, increases in magnification show more stars. If you used 10x, 12x and 15x binoculars on the same star field, you would see 25-50% more stars with the 12x and another 25-50% more again with the 15x. If you tried to get the same star magnitude gains by increasing aperture and keeping power constant, the size increases over 50mm would quickly make the aperture diameter prohibitive.

Whether large aperture low power binoculars are the right choice in binoculars depends very much on what you intend to use them for.


Relation: aperture-mag when view extended objects

In my 15x70mm Binoculars I know that I can see more stars than a smaller pair, but what about things other than stars?

Suppose I'm looking at M42 with my 15x70's. I assume that magnification doesn't matter too much as it's already pretty big, aperture big enough to match the magnification is more important. I will rate the 'brightness' of M42 through this pair of binoculars as 1.

Now suppose I use a pair of 7x50's. I had thought that these couldn't compare to the 15x70mm's, but now I'm not so sure. If I was to look at M42, it would look smaller, but also stand out more, right? I'm only getting half as much light as I was with the 70mm lenses, but I'm also only magnifying it to have an area about 1/4 the area of the 15x70's. So I would give these a brightness rating of 2. Which is twice as good as the 15x70's.

I know that the bigger image scale does make up for the dimmer view somewhat, but how much does this matter?


There's a lot of factors that determine whether an object will stand out more in a lower power larger aperture binocular.

The larger aperture has greater light gathering ability.

Image scale has a lot to do with whether you can see detail in an object or not.

Extended objects, especially faint diffuse objects benefit from greater aperture.

In binoculars, even globular clusters and dense open clusters can be considered extended objects. These will benefit much more from larger image scale provided by magnification.

I've seen the North America nebula with my 15x70s. I've never seen it with my 10x50s. Assume viewing a 1 arcminute square. If you consider the amount of light gathered spread over the area of the magnified image, then the 15x70s vs 10x50s gather 1.96x the light (70x70=4900 vs 50x50=2500, so 4900/2500=1.96) and they spread it over an area 2.25x as large (15x15=225 vs 10x10=100, so 225sqarcmin/100sqarcmin=2.25x). If 1.96x the light is spread over 2.25x the area, you would think the 15x70s would make it more difficult to see, but that is not the case. Larger image scale can sometimes make it easier to see faint light.

edz

Read the full response at this post
Relation: aperture-mag when view extended objects?


EXIT PUPILS

I wanted to test the difference between two perfectly matched binoculars, but with one variable, exit pupil. I tested 16x70 Fujinons and it's twin, the Fujinon 10x70. Not identical twins, but sure from the same family. These two binoculars share exactly the same everything except the eyepieces. So, only power, and hence exit pupil, are different (and fov, but not important here).

Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. Consider it to be a grey scale. You all understand the affect of dark sky background. Higher magnification in a telescope reduces the extended light of the background until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers more light, not only from your subject, but also from the background. This can be beneficial or detrimental, depends on the condition. Generally, this makes it much more difficult to see the subject, unless you are viewing under very dark skies.

In moderate 4.5-5.5 skies, 10x70s show a background about three or 4 shades brighter on the grey scale than what is delivered by 16x70s. With a 7mm exit pupil, there is so much light coming into the eyes from the extended sky background, it makes it more difficult to see anything and everything. The 16x70s provide such a contrasty view against the darker backdrop, the contrast gain from the higher magnification of the 16x70s was far reaching and unmistakable. These are exactly the same binocular, so the only improvement in grey scale background most definitely comes only from the increase in magnification, and hence a smaller exit pupil.

But on one night, I did get both binocs out under mag6+ skies. Since the sky was already so dark, the backdrop in the 10x70s did not appear so much brighter than the 16x70s. Much more could be seen with the 10x70s in a darker sky than in the bright sky. Faint nebula stood out now that they were being used in their productive element. On this night the difference between the two was not nearly as great. The 16x70s still see more, but on the faintest nebula, the 10x70s did very well.

My point is not to say one is better than the other. For me, in most conditions, the 16x70s will perform better. But those 10x70s, under the darkest skies I can get to, would be just the tool to observe extended objects and deliver that "finishing" light to my eyes. They have their place. But used in the wrong circumstances, they are no match for the right tool.

Every tool has it's purpose.

edz

Exit Pupil, Large or Small


sure...more magnification means more diffusion of the extended object's light, but I'm asking about the explicit relationship between exit pupil and brightness, not magnification and brightness per se. yes, these are related, but they're not the same thing (keep in mind that two different scopes/binos can give you a different exit pupil for the same magnification...or the same exit pupil with different magnifications).

If we were looking at a white diffuse surface with equal instruments except for exit pupil diameters, the one with the larger exit pupils would have a brighter image. One of the big issues with large exit pupils is that under light-polluted skies they transmit the light pollution as well as the celestial objects. The claim is that smaller exit pupils yield more contrast by reducing the impact of the light pollution

In order to determine the brightness of a view through a binocular you not only need to know the exit pupil size, but also the AFoV. A binocular with a larger exit pupil doesn't *always* give more light to the observer.

Well, not according to the official definitions I've read.

AFOV has nothing to do with image brightness. You are confusing the image formed in your eye with the exit pupil. The exit pupil is simply the image of the effective aperture of the system.

How much light a system transmits is related to its effective focal ratio. If you change the magnification of a system, you also change the effective focal ratio. This has nothing to do with spreading light - sky glow has the same luminance value whether you measure a tiny area or a whole slab.

Lets see how this works. We will use telescopes because we can change the eyepice. The OTA has a printed focal ratio. The brightness of the aerial image formed is dictated by the focal ratio of the scope. An eyepiece will be used to make an image we can see - in astrophotography this is known as afocal projection but using a camera rather than an eye.

The effective focal ratio of the system = OTA focal ratio X eyepice projection magnification.

The eyepiece projection magnification = the eye's focal length / eyepiece focal length.

As you can see, the magnification increases as the eyepiece focal length deceases. And with the first equation, the systems focal ratio increases with the magnification. Using shorter focal length eyepieces increases the effective focal ratio of the system and makes darker images. As the focal ratio increases, the exit pupil deceases.


Light output is not controlled by field of view. If I use an eyepiece in my scope that produces a 3mm exit pupil, it doesn't matter if that eyepiece has a 50° Afov or a 68° Afov, the light output is exactly the same. I just get the same light output over a wider area.

So in effect the view is being dimmed by having a wider AFoV? If you were to somehow block off the edge in the 68 degree AFoV so that all I see is 50 degrees I would assume light output is decreased so that it is less than the orginal 50 degree view?

Again, Light output is not controlled by Field of View.

What is "image brightness"? Can anyone give a clear definition?

several links to sites

One definition of "Image Brightness"

The apparent luminance of the image as seen through an optical system. This brightness is determined by the brightness of the object, the transmittance and the diameter of the exit pupil of the system.

Take a specific magnification, say
10. Then in relation to that, the larger objectives (gathering
more light) yield larger exit pupils:

10x30 = 3
10x50 = 5
10x70 = 7

So the brightness is directly expressed by the exit pupil: the
ratio of light-gathering to magnification.

In terms of the brightness, the largest exit pupils can only be
fully used by dark-adapted younger folks on moonless nights
under dark skies, because the sky background is very dark in
that situation. But under moonlight and light pollution, then
having a lower ratio of gathered light in relation to
magnification yields more contrast, which makes fainter stars
easier to see, since the background is darker.

Brightness in Exit Pupil
Why some exit pupils appear brighter than others of equal size.


Ray Diagrams
All the rays from the cone passing thru the objective to the focal point will pass thru the eyepiece. No rays will be lost. The cone of rays from the objective focal point to the eyepiece exit pupil has a different angle cone for every eyepiece. The shape of the cone of rays gets narrower or wider depending on the eyepiece focal length. That's what focal length is all about. Focal length of a lens creates a cone of light with an angle from the apex at the focal length to the base at the lens. A ray diagram showing a cone of light from the focal length of the objective, once the rays pass the focal point, will show rays at the angle of the eyepiece focal length. Of course F obj / F ep = Mag.

Focal length of a lens


Edited by EdZ (02/13/10 09:44 AM)


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Resolution and Visual Acuity new [Re: EdZ]
      #49491 - 02/03/04 09:10 PM

Here's a link to resolution tests that may help put to sleep the many posts on the internet that display data (not listed as test results, just posted as unqualified data) that states to the effect that a 10x binocular can achieve 6 arcseconds resolution or even an 8x binocular that can resolve 4 arcseconds.
Binocular Resolution Testing w/USAF Charts

The amateur astronomer should know the Rayleigh Limit of a lens determines it's resolving limit, however, the resolving limit can only be achieved at magnifications on the order of 25x per inch of aperture. Sure a 50mm lens has a rayleigh Limit of 138/50 = 2.76 arcseconds, but you have no hope of ever seeing that in a binocular.

Also for astonomical use in dark skies (scotoptic light) the human eye is capable of only achieving 2x to 3x less resolution than in daylight (photoptic light). A retailer that is reporting a resolution of 60 arcseconds for the human eye and 3 arcseconds for a 50mm binocular or 6 arcseconds for any 10x binocular is giving grossly misleading information.

Also the amateur should be aware that the USAF Line Pairs Chart is testing resolution in daylight on high contrast line pairs. The results would be much better than anything that could possibly be achieved in astronomical use due to the ability to see certain kinds of resolution (line pars versus point sources) differently. No binocular can reach the same results on point source resolution under dark skies.


Other Resolution Questions

Below is a series of questions and answers that have been culled from various posts here. There are a number of valuable links that will allow you to godd read some important discussion concerning resolution.


Q There's a strong chance that I will be purchasing Oberwerks' 22 x 100 giant binoculars or something very similar. I just want to know if the 22x to 25x magnification found on most giant binocs is enough to resolve globular clusters? and I mean resolve not grainy...

A I would be surprised if 22X binos could resolve globular clusters. I can't resolve stars in even M13 before about 100x with my 8in reflector.

My 150mm refractor needs 200x to resolve M13 to 60 stars. With my 20x80s I cannot resolve any globulars, although I look at a lot and still enjoy the view.

Q These (25x100mm) bino’s have a good solid build. ...M22 had a mottled look, and it was very easy to see that it was a globular cluster even though there was no way of resolving it at 25x.

A For anything less than optimum magnification, resolution delivered is dramatically reduced. ... Your 8x42 BEST binocular may be capable of 10 to 12 arcsec, and a 16x70 Fujinon is capable of about 7 to 8 arcsec, by the best observers. For astronomy, you cannot achieve those results.

A ...as magnification increases we get to resolve easier. For example, an individual with 150 arcsec acuity can use 10x binoculars to resolve a 15 arcsec double star. However, the same person with 150 arcsec acuity would not be able to see a 1" arcsec double resolved if magnified only to 150x. It would take more like 200x or 250x or more. As objects get closer to the Rayleigh limit, resolution becomes far more difficult and a much higher magnification is need.

Resolution of binoculars


Q Being as I struggle to come to terms with measurements such as "arcseconds", I wondered how close to these miniscular measurements comes a top "E" first guitar string ,measuring precisely 0.3 millimetres, being "clearly resolved" through 10 x 50 binoculars from a distance of 100 metres in very unfavourable lighting ?

A 0.6 arcseconds. That would be near the equivalent of seeing the Cassini Division with those binoculars. That falls within the realm defined for extended objects of line shape.

What happens when the thin line you've selected reflects light? Two very important conditions conspire to allow you to see this thin reflective wire.

First, It becomes similar to a long line of point sources. The light from a point source can be seen even if it has imperceptible width.

Second, A lens is not capable of showing any object smaller than it's limit of resolution. What it will do is make anything smaller than the Airy disk appear as the same size as the Airy disk. Your lens has actually fattened up the image of the line...

Resolution and contrast are related. The lower the contrast of the target, the lower the resolution of the optical system. Brighter stars are easier to resolve. Shiney wires are the same. Most resolution tests are done with high-contrast targets not only because they are easier, but also they give better numbers.

Resolving a thin wire



Visual Acuity is a limiting factor in the eye’s ability to achieve resolution and magnification is employed to make an image large enough for the eye to perceive. If your visual acuity is 200 arcseconds, you need 12x mag to see a 17” double or 20x mag to see a 10” double.

Acuity can be measured as the apparent separation results for a range of varying doubles recorded with one or more pieces of equipment. The results will fall into a fairly narrow range. This would be the observer's acuity range.

Measuring visual acuity is not the same as measuring the resolving ability of your instrument. Resolving is the closest components that can be detected. Acuity is the apparent separation results for a range of varying width doubles measured with various equipment, or even measured with one piece of equipment on various targets. It is surprising how the apparent separation results for a variety of doubles will fall into a fairly narrow range. This would be the observer's acuity range.

Humans are not capable of 1 arcmin resultion in astronomical viewing applications. This value represents the absolutely very best vision, (think O'Meara here), observing a black line on a white ground (think the power line example), in the very best optimum lighting conditions (think black line against blue sky on a bright sunny day with no glare).

...any number of publications report tests that show that a better than average human is capable at best of 2 arcmin acuity, ...and normally 3 arcmin or less of resolution when viewing point sources under subdued light conditions (astronomy).

Testing for Visual Acuity

Q Are large aperture binoculars providing greater resolution?

A Most people will argue that the size of the objective lens is what determines the resolution, and yes this is true. But how much of that resolution the eye can see is dependant on how large the resolved image is magnified. Just about every binocular objective size provides more than enough resolution for the eyes. However, without sufficient magnification, the eye will not see what has been resolved.

...even a 50mm (2") objective is capable of resolving two stars 3" apart. But it would be resolved only if you magnified it large enough for the eye to see it. There is not a common fixed power binocular on the market today that will allow you to see the split in a 3" double, even the 25x150s. But if those lowly 50mm binoculars had 60x or 70x magnification you would be able to see that 3" double as split.

This thread discusses some of the reasons for large aperture binocs
Are large aperture, low mag binoculars pointless


Here you will find explanations for some of the terms used to define resolution
Rayleigh Limit / Dawes Limit


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EdZ
Professor EdZ
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Reged: 02/15/02

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Terrestrial Viewing new [Re: EdZ]
      #50125 - 02/05/04 06:23 AM

Do the critical specs/features to look for when buying large binoculars change when the dominant usage will be distant terrestrial viewing, or do the same factors apply as for astronomical observing?

Large binoculars for distant terrestrial objects

...you mentioned looking across considerable expanses of land & water. In the daytime, frequently there is considerable heating or cooling of the air (what the meteorologists call heating-degree or cooling-degree minutes). This in turn causes thermal activity, which causes the kind of shimmering "funhouse effect" to the image that you see in the movies when a scene is filmed in the desert...the higher the magnification, the more pronounced it is. I currently have 4 hi-end binos and a premium scope and do a lot of birding by local lakes. I find that very seldom can I use the really hi-power eyepieces on the scope and often even the stronger binos don't work as well as the lower powered ones when there is several degrees difference between the air temp and that of the water surface, or between air temp at ground level and just up a few meters. Air has to very stable to have good seeing conditions if you're talking about looking at something much over 2-3 miles away and seeing it to best advantage. I can only use the highest power eyepiece on my scope (48x) very seldom and get the kind of image it's capable of producing...usually the lowest pwr ep (24x) will give a cleaner, more usable view with just as much detail, just a smaller image. This is one of the problems of terrestrial viewing vs cosmological....when stargazing you're looking thru much less dense air which isnt as susceptible to thermal currents


additional posts will be added






Edited by EdZ (02/13/10 09:42 AM)


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Edge Performance - Sharpness of Field new [Re: EdZ]
      #50807 - 02/06/04 11:48 AM

In most binoculars, the image deteriorates the further out from the center of the field of view. Some binoculars have special lenses that help alleviate this problem, and some are just better than others. In general the wider the field of view, the more distortions present at the outer edges of the field of view.

There are many threads where a brief comment is made about edge sharpness, but this one thread covers it in detail. Here is a good discussion related to edge performance.

Edge Performance

Also, you must recognize that edge performance, as noted in a concise statement made by Kenny in the thread above, "["Edge performance" is in itself a rather inadequate title to cover what I consider to be an attempt to categorise what could encompass a whole range of possibly quite different and unrelated properties of an optical system.]", can be caused by a number of issues. To understand better the causes, you should also read the Best Of thread titled
Binocular ABERRATIONS AND DEFICIENCIES


Another thread related to the same topic, once again initiated by Kenny J
True ( SHARP ) field of view


See also this table of measured Binocular Field Sharpness for approx 30 binoculars
So often we hear brief reports about new binoculars to the effect, "this binocular is pinpoint sharp from edge to edge", or "I can only see distortion in the outer 5%-10% of the fov". How are we to believe subjective statements that are not supported by accurate measurments? Frankly, I have tested over 35 different binoculars and have found no fixed power binoculars that are sharp edge to edge and I have found only 2 or 3 that do not exhibit undesirable un-sharpness beyond 80-85% out. With well corrected eyepieces in variable power Binocular Telescopes, you can achieve edge to edge sharpness. This table shows results for a standardized means of measuring that sharpness across all brands of binoculars. It eliminates the subjectiveness.

edz

Edited by EdZ (02/13/10 09:41 AM)


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PRISMS - BaK 4 vs BK 7 - Roof vs Porro new [Re: EdZ]
      #50808 - 02/06/04 11:49 AM

The original discussion that took place concerning the glass type used for prisms is buried within a thread where you would never find it. Here is a link to that discussion. Not all posts in the thread relate, so you may have to skip around a bit.

Where to buy now retitled to Bak 7 vs Bak 4

And don't miss this post way at the bottom of the thread
Bak 7 vs Bak 4 when to use


Another thread with some additional comments

Bak6?


This more recent thread (9/05) discusses some of the properties of BaK4 and BK7 glass and why they are (or are not) used for prisms in binoculars.
Prisms and Coatings - Differences


Roof Prisms vs. Porro Prisms
What are the advantages or disadvantages of one over the other, or are the differences just perceived differences? Do the different types of prisms have any affect on Depth of Filed or Stereoscopic vision? This thread begins to discuss the issues. Dut also refer to the Best Of thread on
Depth of Field and 3D or not,3D! for more related to the same topic.


Edited by EdZ (02/13/10 07:24 AM)


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Binocular Mounts - Tripods - Adapters - Motions - Homemade new [Re: EdZ]
      #51289 - 02/07/04 10:17 AM

We don't have a Minireviews post set-up to list mounts, but this thread is as close as it gets. There are over 30 links here to mounts and tripods threads. some of those threads have 3 or 4 or even 10 more links. Many of the posts you will find are much like mini-reviews of the tripods and mounts mentioned. Also included here are some links to full length articles over on the CN Rreports side. This post is your comprehensive index card linking you to Binocular Mounts. edz


A CN Report on Binocular Mounts, Tripods and Mounting Accessories
http://www.cloudynights.com/documents/thoughts.pdf
pick the right tool for the job. Enjoy. edz



And if you just want to know,
"What are the motions of a binocular parallelogram mount?",
then check this out, it explains the 6 degrees of motion.
Binocular Parallelogram Mount Degrees of Movement



Amateur Made Binocular Chairs and Paralellogram Mounts
Probably the single most comprehensive list of links to Amateur Made Binocular Chairs and Paralellogram Mounts (collected from 60-70 locations, and growing) that you are likely to come across anywhere on the internet. This thread includes a good collection of advice based on lessons learned on "How To" make some of these mounts and chairs. Thanks to Craig Simmons for this collection.
Amateur Binocular Mounts


Many times people ask, Why do you always say you can't mount an 8# binocular on a tripod that is rated for 8#. That is because the load rating for tripods is determined with the load weight directly over the tripod head and directly over the tripod column. Furthermore, load ratings are based on cameras with little to no magnification. With binocular viewing, we often have a big load hanging off of one side. That is like a cantilever or an eccentric load. In all things mechanical, it takes a beefier support to hold an eccentric load than it does to take a direct load. Add to that some 10x or 12x or 25x magnification and you can see the need for stability goes up exponentially.
Binoculars Create Eccentric Loads on a Tripod


Mounting Giant Binoculars on a Budget
There are a lot of different suggestions in this thread. They are not all low budget. But there is a wide variety of suggestions made by all the various users, how they view their performance, and a lot of links to various types of products. very good information here.


Binocular Mirror Mount


Neck Monopod Review


I'm looking for a tripod for my binocular. Which tripod is good for holding a binocular up to 25x100?
This question comes up once or twice a month. here's some advice. And links to more advice.
Tripods?



A 7.5# 20x80 mounted on a Bogen 3011 tripod / 3130 fluid head.
Is this a good combination?


Binocular Mounts and Tripods - Farvorite Combos
Several users post what they like to use best as favorite binocular mount combinations


Excellent summary of differences between using photo tripods, parellograms, binocular mirror mounts and binocular chairs. Also, excellent summary of various models of Bogen legs and heads , how tall they are, their weight capacities and some links on where to get good prices.
In addition, references to other tripods and links to articles.
Binocular Tripods


Bogen Manfrotto Models for Heavier Binoculars This discussion took place around selection of a tall tripod to hold 16# load. The recommendations are for big heavy duty Bogen models.


Very good comparison of two models of Bogen tripods. Lots of pictures
Bogen 3051, 3046 side by side


Manfrotto-Bogen Cross Reference




Here's information about the use of the new Universal Astronomics Microstar
Microstar Bino Mount


Here's some info on the popular Orion Paragon Tripods
Orion Paragon or ??

Orion Paragon Tripods

How to Make a Paragon Binocular Mount Adapter This post describes a way to construct your own mount adapter for the Orion Paragon Plus binocular mount. The techniques shown here may have application beyond this specific situation with some adaptations.



This particular post is located in the Mounts Forum but it includes about 10 links to discussions on
Bogen/Manfrotto tripods, Orion Paragon tripods, UA Microstar Mount and a few others.
Manfrotto 3130/3011N for PST, ST80? Other Bogen?




A variety of tripods are listed here with user experiences. It's a good thread to compare one to the other. Must be 10 tripods list in here, everything from department store camera tripods to a link for a full length article on what to look for in a parallogram. Users note some instances where a camera tripod might be all that's needed and some of the pitfalls of using camera tripods for heavy binocular mounts.
Need a sturdy tripod, what do you recommend?


Why don't we see Equatoral Binocular mounts?
Very few positions would ever afford you a level eye position. You would almost always have your head tilted to one side.
mounts and binos


converting a surveyor's tripod to take a mount for a binocular
tripod help


tripods, parallel mounts, and a skywindow for binocular astronomy, some subje...

experience in search of the best tripod/fluid combo for astro binos


What do some other people use as their binocular mount setup? Putting together a versitle parallelogram setup.
Help with a new binocular setup


Home made binocular chairs and parallelogram mounts. Lots of pics.
Observing chair/mount for binos or scopes


One member comments "...I think the 3046 is the best "non-hiking-capable", "budget" astronomy tripod in the Bogen line at around $200. The 3265 head serves its purpose, but I'm now sure that a parallelogram mount is in my future"
Tripod & head report


after a month and a half search, this member chose the Slik 700DX Pro. There were atleast a half dozen other choices offered up as suggestions. Here's the results. Read the rest of the thread for all the other suggestions.
Report: Slik 700DX Pro Tripod



Comments on Binocular to Tripod Adapters

IMHO, the two best tripod adapters are Orion's deluxe (search in Orion accessories), the simple tall L bracket and Universal Astronomic's deluxe. UA's is by far the best. Orion's is thicker metal and taller than most simple L brackets. UA's deluxe is even thicker than Orion's deluxe and it allows the binocs freedom of movement to tilt a little left right. Orion's sell for about 18.95. Ua's sells for $29.95, but it's worth it..

20x80 binoculars are too big for a small L adapter. I'd advise picking up a tall heavy duty binocular L adapter, not just a short thin one. That will help eliminate shakes.

Most large 20x90, 22x100 and 25x100 binoculars do not require a separate adapter. The center mounting post is the adapter.

edz

Orion adaptor

Pentax adapter

Window adapters and tripod adapters

Celestron's adapters, one's metal, one's plastic

comments on plastic adapters

an assortment of adapters, metal, composite and plastic

Nikon 12x50 SE Tripod Adapter


...high quality binoculars in that aperture/magnification range (i.e., the Doctor 15x60 Nobilems, the Zeiss 15x60 B/GATs and the Nikon 18x70 Astroluxes) all use non-standard tripod adapters. The one that Nikon sells for their Prostar and Astrolux binoculars also works for the Zeiss 15x60s and costs less than half as much as Zeiss's tripod adapter. The Doctor Nobilems use a metal tripod adapter that is unique to their binoculars.

Minireview of the Adorama adapter not the favored choice here.

What? No tripod adapter screw hole? Try a strap mount.
Eagle Optics Plate with Velcro Strap mount.

Tripod mounting an old bino with no socket

Narrow body L adapter for mounting Oberwerk Ultra / Garret Signature
standard wide brackets will not work with thiss model binocular. Of course they both come supplied with this bracket, but it's a good one.

a website offering 20 different kinds of adapters
not necessarily all the cheapest offers, but just look at the variety!

FarSight Binocular Mounting Bracket


A not on using a laser pointer on a mount
Here is the paragraph from the FDA's guidelines on laser pointers:

"Laser products promoted for pointing and demonstration purposes are limited to hazard Class IIIa by FDA regulation.

21 CFR 1040.11(b) and 1040.11(c), limit surveying, leveling, and alignment, and demonstration laser products to Class IIIa. This means that pointers are limited to 5 milliwatts output power in the visible wavelength range from 400 to 710 nanometers. There are also limits for any invisible wavelengths and for short pulses. Pointers may not exceed the accessible emission limits of CDRH Class IIIa or IEC1 Class 3R."

Lasers of greater power than 5mW are designated as being Class IIIb lasers and, therefore, are not legal for use as pointers.

How do you mount your BIG binoculars This is a collection of posts showing and discussing various mounts.

Edited by knuklhdastnmr (09/14/13 10:24 AM)


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How to Focus Binoculars new [Re: EdZ]
      #51290 - 02/07/04 10:18 AM

It can be surprising how many people new to using binoculars, or even some who have had them for a long time, do not know the correct procedure to focus binoculars. At every session I teach, there are always a group of people that do not know how. Erik D recently posted this procedure and it was a big help. I'll repeat the post here, but first; A few other things users should know about focusing:

Of course, this relates to binoculars with a Center Focus wheel that moves both eyepieces and a Right Diopter adjustment that moves just the right eyepiece.

If you have binoculars with individual eyepiece focus, just use each separately as if each were a diopter adjustment. Actually, that's exactly what it is.

Best focusing can be achieved by focusing "IN". So start with the eyepiece turned out and "FOCUS IN". Stop when you hit a pinpoint image. Rocking back and forth over the best focus point is not recommended. We have decades of military testing to thank for that little tidbit.

And now from Erik D.

1. Cover the right objective.

2. Look thru the binos while keeping both eyes open and RELAXED.

3. Use the center focus wheel ONLY to achieve sharpest focus. Do NOT try to compensate for out of focus image with your eyes. Let the focus wheel do all the work. Best focus is achieved when star image is the smallest pin-point. This is the same process as if you are looking thru a single EP of a scope. It should be pretty easy to tell if you have achieved best focus because you'll see spikes and flaring of star image otherwise.

4. After you have achieved best focus with you left eye uncover the right objective and cover the LEFT objective.

5. Using the Right Diopter Adjustment (RDA) on the right EP ONLY, focus the image thru the right barrel. Remember to keep both eyes OPEN and RELAXED. Only the RDA should be doing the focusing, not your eyes. Do NOT touch the center focus wheel while your are doing the RDA.

6. When you have achieved pin point focus in your right eye you can remove the cap on the left objective and images in both eyes should be in focus.

The goal of this procedure is to use the RDA to compensate for any difference in eye strength between the left and right eye. You do not have to match them or make them "equal" (by setting them both on the same number).

Make sure your binos are on a solid mount/rest before you attempt this procedure. No one can hand hold 8.8 lb binos and perform critical RDA at the same time.

Erik D

Thanks Erik.




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ABERRATIONS AND DEFICIENCIES new [Re: EdZ]
      #64093 - 03/02/04 06:49 AM

Nikon provides a brief explanation of the six major aberrations here
Nikon Sports Optics - Aberrations and Deficiencies

Vladimir Sacek provides a much more in depth discussion of the major aberrations in his web article here
AMATEUR TELESCOPE OPTICS See INTRINSIC TELESCOPE ABERRATIONS

Tom Trusock wrote a paper with some easy to undersatnd explanations
Telescope Optic Aberrations


MAJOR ABERRATIONS

ASTIGMATISM
Astigmatism makes it difficult to ever achieve a precise focus. The telltale sign of astigmatism is found when you view the out of focus Airy disk image. On one side of focus it will be elongated in one direction. On the other side of focus the elongated orientation of the image will change by 90°. There can be several causes of astigmatism. One of those causes can be a miscollimated or misaligned optical tube. It can be in the eyepieces. It can be in the objective lens. If the optical axis is perfectly aligned and the if the images are merged between the two barrels and astigmatism is still present, the problem is not one the user can fix. At any rate, the affect of astigmatism is that you will notice elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. The affects of astigmatism are generally eliminated with high power and small exit pupil, but that is usually not achieved in binoculars.


SPHERICAL ABERRATION
Spherical aberration is a displacement of the rays along the length of the optical axis. Rays originating in the lens further off axis do not reach the same focus point as on-axis rays. If present, even on-axis images may look considerably soft and no amount of refocusing or flattener will help to improve. The best you can achieve is focusing on what is known as a least circle of confusion.

Stopping down or masking the outer edges of an objective will help suppress spherical aberration. This would have the effect of eliminating improper focal length rays generated at the extreme outer portion of the lens. This would improve or "reduce in size" the least circle of confusion at or near the focal plane. This would give a more pinpoint image to focused stars, but, at the expense of lost aperture.


DISTORTION and FIELD CURVATURE
Q - I was hoping someone could explain to me the difference between distortion and field curvature? At the very edge of fov in my binocs I am noticing the stars stretched out a hair compared to very tight dots in the center to about 90% out.

A - By Bill Tschumy
Distortion is of two types: barrel distortion and pincushion distortion. It isn't related to sharpness of the image, but rather image scale across the FOV. If you look at a square centered in the FOV, with pincushion distortion, the sides of the square will be bowed inwards. With barrel distortion, the sides will bow out. Generally, when looking at a star field, the distortion is not much of an issue. Sometimes, when panning, barrel distortion can give a "rolling" effect that makes some people dizzy.

Field curvature is when the outside of the field of view has a different focal point than the center. You can't get both in focus at the same time but you can get one or the other in focus.

What you describe as the stars being stretched out near the field sound more like coma to me, yet another aberration.


Bill T dedcribed distortion and curvature pretty well. As Bill said, "It (distortion) isn't related to sharpness of the image". Distortion does not affect the image of the star, it simply moves it in the field of view. Generally it cannot even be seen in astronomy, but can usually be seen when the binocular is used for looking at terrestrial objects where we are accustomed to seeing nice straight line edges of things like buildings and poles.

Pin Cushion Distortion discussion explaining reasons for pincushion

PINCUSHION DISTORTION IN BINOCULARS drawings showing pincushion


FIELD FLATTENERS TO REDUCE CURVATURE
Curvature of field (change in focus off-axis)can be reduced by using a field flattener lens. Without the flattener, the outer portions of the field will be slightly out of focus when the center is in focus. Instead of pinpoints in the outer field, this will have the tendency to enlarge and blur the circles of light that form the stars in the outer field. They would be slightly out of focus. (Slightly enlarged circles, that is assuming no other aberrations are present).

A field flattener can be used to insure that the entire field is in focus at the same time. Stars in the outer field will be focused and appear just as stars in the center (again, assuming no other aberrations are present). The field flattener does not eliminate spherical aberration, astigmatism, coma or chromatic aberration.
Field Flattener vs Field Aberrations


Field Curvature Affect on Depth of Field
Of the aberrations mentioned above only one can be changed by refocusing, field curvature. Field curvature appears because the outer portion of the field of view may have a different focal point than the center. The focal length increases as you move further out in the fov. You can't get both the center and the edge of the fov in focus at the same time but you can get one or the other in focus.

An astronomer would prefer all binoculars to have as little field curvature as possible so the entire plane of the fov has as little distortion as possible. It is not unusual to find that a binocular with much less sharpness out at 60 or 70% in the fov has a much greater amount of field curvature than a binocular which appears sharp out to 80-90% of the fov.

A terrestrial viewer might want to consider how field curvature affects the view, and may in fact find field curvature to be a useful aberration. While it does nothing to add to depth of field further distant than the plane of focus, field curvature provides for closer objects to appear in focus as they range out across the fov.

It seems clear from this correlation that the binoculars with greater field curvature have the perception of greater depth of field in front of the focus point. Therefore, it may stand to reason, a binocular with more field curvature may provide a greater “near” depth of field for the terra user. (of course that won't do you any good if all the birds you want to see are in the same tree sitting on branches between 45m-55m away). Also, it may be that a field flattener lens, which would be desirable for an astro viewer to get as flat a field as possible, might work against a terrestrial viewer looking for the greatest depth of field.

Curvature Affect on Apparent Depth on Field



COMA
Coma is generated by off-axis light rays. The intersection of light rays from different points along the objective is not symetrical. Coma is present when you see images of stars begin to spread out in a fan shape. Rather than see a tight spot of all the rays forming the image, a diagram would show most of the rays forming the bright point and the remainder of the rays spreading out from that point to form the recognizable fan or comet shape. Coma increases toward the outer edges of the lens. A star will have its brightest point towards the center with the fan shape extending outward.


CHROMATIC ABERRATION
this topic has been moved to its own thread

All achromats show some CA. It is a function of lens design. It is also a function of focal ratio. Very few binoculars are designed specifically to suppress CA. BTW, CA is not considered a major aberration. Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Lateral CA increases as you view objects further off axis and is more pronounced towards the outer edge of the lens. Lateral CA is seen as the outer edge of the object bleeding, usually a different color, generally observed in large bright extended objects. Significant CA (on the moon or planets) may cause some edge softness on both and loss of contrast, but it should not be confused with flaring.


Quote:

Many of the aberrations we see in binoculars affect edge performance. In fact, the aberrations described here all look somewhat different and affect different areas of the view.

WHAT DO ABERRATIONS LOOK LIKE

ON_AXIS
Generally, not all aberrations are seen on-axis. The on-axis image created by aberrations is as follows:

curvature not seen on-axis.

coma not seen on-axis.

If the stars seem never to focus to a fine point, but at best remain slightly bloated and are still circular, and it cannot be focused out it is probably spherical aberration.

If the star seems to not come into sharp focus it may be astigmatism. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. Passing in and out of focus will show the out-of-focus image to flip orientation by 90°.

Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Sometimes seen as a small circular band of color areond a finely focused star point.

OFF_AXIS
As for how you notice stars in the outer fov, the image created by aberrations is as follows:

If the star seems slightly bloated, but it can be refocused down to a finer point, then it is curvature, as Bill T described.

If the stars seem flared as wedges with the point towards the middle and the flared wedge toward the outer edge, them it is coma. It cannot be focused out.

If the stars seem slightly bloated but are still circular, and it cannot be focused out it is spherical aberration.

If the star seems slightly bloated and slightly elongated than it is astigmatism. Astigmatism will show elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint.

If you have both astigmatism and coma, the off axis images will look like comets with an oblong axis, or like seagulls.

Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point.

Lateral CA will produce color fringes around all bright objects when viewed slightly off axis, but you should see a different color towards the lens center than you see towards the lens edge.







Spherical Aberration usually is referred to as an aberration in the optical elements. However, when referred to in the exit pupil, usually what is meant is "Kidney Bean". Read the explanations here.
Spherical Aberration of the Exit Pupil


Binocular deficiencies to watch out for:

GHOST IMAGES
Coated vs Multi-coated vs Fully MC will have an affect on total light transmission and contrast. Lack of coatings may produce ghost images internally. Ghost Images and internal reflections is a deficiency that will destroy contrast.

PRISM LIGHT CUTOFF
The following post has a photo of a binocular through the objective lens. look closely at the image and you can see light blockage at the edges. This is known as prism light cutoff. The net effect = this is reducing the effective apperture of your binocular.

Here's the photo of prism light cutoff
Prism Light Cutoff shown in photo

In this image above here you see the perfect example of prism light cutoff.

This is not related to collimating binoculars. This is not what you would see if one side or both sides of the binocular were out of collimation within the tubes themselves. In that case you might see ovals or cats eyes or the images would not be merged.

In this picture, either the prism housing infringes in the path or prisms themselves are so large that they infringe in the light path, or the prisms are to small to accept the entire light path from the objective and you see the edges of the prism cutting into the light path and cutting off the edges of the exit pupil.

Here's a link to the discussion and the description of what is going on in this binocular system
Prism Light Cutoff ...

Here's a link to another discussion as relates to another model binocular
Prism Light Cutoff - What to look for - How to calculate


COLLIMATION
A binocular that is out of collimation will show double images. It is corrected by mechanical adjustment.
This has been moved to it's own post.


FLARING
Q - I see flaring of the stars in my binocular? What is it?

A - I'm not sure what you mean by flaring. In fact I think people have different meanings for this term. I think some people are referring to radial spiking and some to ghosting. Flaring in photographic images refers to something similar to ghosting, as in lens flare. There are other things people might mean when they say flaring.

What some people are calling flaring might be lateral chromatic aberrtion. Lateral CA increases the further off-axis you view the object and is more pronounced towards the outer edge of the lens. Lateral CA is seen as the outer edge of the object bleeding, usually a different color than the inner edge, generally observed on very bright stars or large bright extended objects and would have a smooth outer edge shaped to the object.

Coma is another good example of an aberration that people might refer to as flaring. Coma would be seen as a off-axis wedge with the point of the wedge towards the center. Generally not noticed on-axis, the wedge would probably get larger as you move the object further off axis. Coma would be seen on all stars.

Severe spherical aberration could be producing an on-axis image that looks like a star point encircled by a diffuse circle of light. The inability to focus all the light into a fine point creates a diffuse glow around the point that cannot be focused out, either on or off axis. This might be interpreted as flare by some people.


However I use the term flaring to mean radial spikes, spikes of light flaring off of the central image disk, more than just a few, and in all directions. The binocular that I had with this problem displayed maybe 30-40 flares coming off the star.
In this thread About two years ago I described flaring as radial spikes.
More recently, the discussion came up again and much discussion was devoted to problems in the eyes in this thread about
Radial Spikes Around Bright Stars


RADIAL SPIKING
On bright stars, I'm seeing a rather nice radial pattern of spikes around the point star. The pattern doesn't flare or move like atmospheric abberations, it remains static as long as I hold my eye still. As I move my eye around the spikes move around as well, but never disappear.
When using both eyes, I cannot get stars to focus to points no matter how I set the diopter and focuser. I have 20-20 vision BTW.

New 20x80 binos - Radial Spikes

The problem Gary describes has nothing to do with mis-collimation nor mis-alignment of any portion of the optical train. Gary describes radial spikes, something akin to radial flaring. This is an anomoly of the lens.

This abberation is similar to some degree to what you might see in a normal lens on a very bright star. Focus on Sirius or Vega and what do you see. In many binocular lenses, there will be what looks like a few, maybe 6 to 8, small diffraction spikes eminating from the bright star. This is usually seen ONLY on the most extreme bright objects.

What Gary has described is radial spikes all around a star. I have seen this aberation exactly as described. This is an unusual abberation that is not normally seen in a lens.

A star test, if viewed on a group of faint 4th /5th magnitude stars would not pick up this abberation. By the time the view moves over to 5th or 6th magnitude stars, there is not enough light for this anomoly to be seen, but it is still there. It will interfere with resolution of bright close objects, especially if the companion is faint, and it will interfere with resolution of extended objects made up of point sources such as very dense open clusters and all globular clusters.

If this is indeed a lens condition, the only kind of test that would detect it is a severe star test, similar to that used to test the diffraction pattern in a telescope. This is not a condition that would normally be discovered by any QC.

I have tested a number of binoculars and looked closely for any appearance in the diffraction image in all of them. At powers of 15x or below, separation in the diffraction rings cannot even be seen. At 16x it can barely be noticed. At 20x the first real definition of the diffraction rings begins to appear.

I would suspect flaring is a result of the figuring of the lenses use in the manufacture. It could be in the objective lenses or the eyepiece lenses. Figuring of the lenses could also be producing soft focus and not appear as flaring.

After much discussion and searching by many people, it seems several tend to agree that a strong contender as a source may be that this problem is caused by pinched optics. It may be a result of uneven pressure during the cementing of a doublet or it may be the retaining ring around the objectice being too tight. Read page 150 of Suiter.

It is also a simple fact that very bright objects produce some flare or minor spikes in the image. Here again, I would expect a more highly figured lens would reduce this flare.



VIGNETTE
Vignette, or the blocking of light, by whatever means, reduces the amount of light delivered by the optical system before it eventually reaches the receptor which in this case is your eyes. There are several different deficiencies that could be causing vignette. Identifying what is causing it is more difficult than identifying how much of a light loss is present. This is a very lengthy thread. I have singled out very specific passages for the reader. Fell free to read the entire 100+ posts if you wish.

methodology of the assessment of vignetting by visual inspection
Glenn LeDrew on Vignetting

Vignette in Binoculars!! YES it's there

Test procedure using a laser light to check Vignette in binoculars

Vignetting in All Binoculars!! the prism is not fully lit by the entire objective

Why can't we see the light drop off?

Graphic representation by field of view

Graphic Representations by % light in exit pupil (see attachment)

Graphic Representation showing affect of Off-Center Exit Pupil on the Overall Light Distribution in the Exit Pupil


EdZ's 22 x 85 GO Signatures
Tests and explanations of vignette that reduces full aperture to some effective aperture.
Pictoral essay showing vignette in beam transfer thru prisms, explaining how to conduct the target laser tests. Shows Illumination and Optical Tilt.
This is the exact same test as was first outlined in the "Vignette" threads above back in April 2004. the testing procedure is identical. The presentation is different. the outcome is the same. Actually I explained how to conduct this same test back on '04, but few people understood it.

Aslo page down thru this thread
Comparing GO22x85, WO22x70, Tak22x60
and you will see photos of several binoculars showing the vignette when lining up the edge of the prism with the edge of the objective. This also mimics the results that you get from the laser vignette testing procedure outlined above.

We often hear people state, exit pupil is what determines the brightness of your binoculars. But stop for a moment and think, that does not explain why sometimes it seems that two binoculars even with the same size exit pupil, do not appear to be equally bright. The above tests that show the extent of the Illumination of the Exit Pupil do explain why one model can be brighter than another. It is not just exit pupil which determines brightness of image. It is also dependant on the maximum extent of full illumination. Internal vignette is the cause of reduced illumination. The thead Comparison of 22x binoculars gives good support to this statement.


Things you should know about "too small" prisms

Comments on use of stops

More on stops


Edited by EdZ (05/02/11 02:27 PM)


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Binocular Articles over on the Reviews side of CN new [Re: EdZ]
      #65290 - 03/03/04 10:55 PM

This particluar post is a bit out of form with all the others here that link to info in the binocular forum. This one links to a binocular articles and reports over on the CN reports / reviews / articles side of Cloudy Nights. These are worthwhile to those searching for information related to binoculars.

How To Measure TFOV Of Binoculars


Bear 15x70 Binocular Minor Collimation Procedures


“Collimating” Binoculars
How to perform conditional alignment with prism screw adjustments


My Comet Couch! How to make your own binocular chair/mount


Globular Clusters For Binoculars

100 Binocular Deep Sky Objects

CN Reviews INDEX to Binocular Reviews

Binocular Observing Report using Four Models

Free Mag 7 Star Charts

Books for Astronomy: For use by novice and avid intermediate astronomy enthusiasts

Binocular Mounts, Tripods & Mounting

More selections of Binocular Mounts including Mirror Mounts

Zeiss Premoistened Lens Cloths


Binocular Astronomy by Crossen and Tirion


Technical Lab Report - Limiting Magnitude in Binoculars

Technical Lab Report - How to Understand Binocular Performance

Technical Lab Report - Affect of Eye Pupil on Binocular Aperture


A Comprehensive review of a 7x35, 10x50, 15x70 and 16x80
includes Minolta Standard 7x35, Orion Ultraview 10x50, Oberwerk 15x70 2002 model, Orion Giant 16x80

A Comprehensive Comparison of Four Binoculars 12x50, 15x70, 16x70 and 20x80
includes Pentax PCF III 12x50, Oberwerk 15x70 2003 FMC model, Fujinon FMT-SX 16x70, Oberwerk Deluxe (original) 20x80 . also includes brief info on Minolta Activa 7x35, Orion Vista 8x42, Nikon Sky & Earth Kestrel 10x50, Orion Ultraview 10x50, Minolta Standard XL 12x50, and Orion Giant 16x80

CN Technical Reports - Giant Binocular Review 100mm Binocular - What Can You See?
Primary comparison is the Celestron Skymaster 25x100 and Oberwerk Giant 22x100 . Also included are Oberwerk BT100 and Oberwerk Standard 20x80.
Brief info on Fujinon FMT-SX 16x70, Oberwerk 15x70 and Nikon SE 12x50.


CN Technical Reports - Small Binocular Review 8x, 10x, 12x Binoculars
Comprehensive review of Nikon Action Extremes 8x40, 10x50 and 12x50
Also includes Swift Ultralite 8x42, Orion Ultraview 10x50, Pentax PCF WP 10x50, Pentax PCF III 12x50, Nikon SE 12x50.


CN Technical Reports - 100MM Binocular Tests and Comparisons
Comprehensive Review of Oberwerk 25x100, Celestron 25x100 Skymaster, Garrett Optical 20x80 Gemini, Oberwerk BT100 Binocular Telescope . Brief Info included on Oberwerk 15x70, Fujinon 16x70, Oberwerk 20x80 Standard

Garrett Optical 25x100 IF Binoculars
If you decide to acquire these make sure you’ve got a mount that will easily handle the ten pounds, do take the time to balance them correctly, and seriously consider adding an index finder. So configured and equipped, they will provide outstanding and memorable tours of the night sky.

Miyauchi Saturn III vs Tele Vue 127
comparison of a high quality 5" APO (TeleVue NP-127) with DenkII binoviewers Vs. the Saturn III. Of course the APO/Denk combination is more than three times the cost of the Saturn III - but just how do they measure up for two-eye viewing at a comparable power? The Saturn III is a long FL binocular (39x100 at F7.5) that has the distinct merits of being very light (only 13 lbs) and offering 45-degree viewing.

CN Report: Oberwerk Ultra 15x70
There are a number of importers/distributors currently offering what appears to be the same binocular. Keep in mind, none of these importers/distributors actually make these binoculars. Oberwerk, Garrett, APM, AP, General Hi-T, TS, AOE, they all import these binoculars, variously referred to as Ultra, Grizzley, Marine, Premium, etc. Thsi comprehensive review discusses all aspects of optical and mechanical quality and compares these binoculars to several other well know 70mm and 80mm binoculars.




Edited by EdZ (01/26/07 04:46 PM)


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If They Look the Same, What Could be Different? new [Re: EdZ]
      #71821 - 03/15/04 07:06 AM

We see this question all the time. "They look exactly the same on the outside. Why would anyone pay more if you can get the same thing for less"? Usually, it's because they are much different on the inside.

One of the things we've been trying to teach people for years is there are a list of some of the differences you CAN'T see. Some are highlighted here:

Contrary to what you might read elsewhere, any shortcuts that can be allowed in the manufacturing process will result in a less expensive product that can be distrubuted for lower cost. Considerable savings in time and process results in a less expensive product, a much bigger savings than a few $$, but one that will not perform as well.

Probably the most important differences in any two binoculars that look the same but sell for much different prices is (A)the mechanical construction, workings and range of adjustment, and (B)(likely more important) the degree to which all the optical surfaces have been figured, polished and coated. There are other things that make a difference in the quality and most of them are not visible on the outside.

If one dealer orders all his binoculars without coated prisms, the manufacturer can simply take the prisms off the shelf and install them directly without putting the prisms through the 3 hour process of the coating machine and then inspection.

It is not the cost of the automated polishing machine that adds to your cost, it is the cost of an individual testing and inspecting each piece of glass before it passes the level of QC demanded by the distributor.

Uncoated or single coated lenses or prisms will considerably increase the amount of light that gets reflected around the inside of the binocular, causing ghost images and reducing image contrast. A fully multi coated binocular requires multi-coating on ALL surfaces. It was discovered several years ago that that the factory claim of fully multicoated was being applied to binoculars that had some multi-coated components and some single coated components. This type of factory claim is still passed on by some importers.

You could be getting lenses that have passed a specific selection criteria for the objectives in one binocular. For another brand binocular, you could get lenses that have not passed the test for brand A, so you take your chances. For a third brand, you may be getting the lenses that did not pass any inspection criteria used to get the lenses in the first two brands noted above. You don't honestly think they throw away all the lenses that do not meet a specified test criteria, do you? Just because they don't meet the criteria for brand A or B doesn't mean they can't be used for brand C. More on this below.

The same binocular body could be fitted out with very different eyepieces and still look exactly the same. I once tested 3 different 20mm plossl eyepieces. They ranged in price from $29 to $89. With minor modifications they could all be fit into the same eyepiece housing. The difference in performance between the three was like night and day. The lowest cost 20mm eyepiece had severe distortion in the outer 40-50% of the field of view and beyond 60% out from center lost a full magnitude to the best eyepiece. The best 20mm eyepiece could still see not only the faintest stars but also very fine resolution at the extreme edge of the field stop.

Sometimes we have binoculars that appear exactly the same, but one model has a much wider field of view than the other. This can be accomplished by simply eliminating the field stop from the eyepiece housing. Of course, what you end up with is a wider fov, but it is not a sharply defined field stop and is generally mush towards the outer edges. Also, this may allow undesirable stray light to enter the fov. Neither the removal of the field stop for the wider soft field nor the stray light improves the image, but it is cheaper to make the eyepieces without the field stop.

The eyepiece lens edges could be blackened or not.
The objective lens edges could be blackened or not.

Shiny or bright metal exposed surfaces on inside of barrels will diminish contrast. Not all binoculars are fully blackened at all internal surfaces.

If a lens has several layers of MgF, then it is multicoated. If so, it may likely not have the characteristic blue color of a single MgF coated lens. A properly multicoated lens surface may reflect only 0.2% to 0.5% of the entering light. A properly single-coated lens surface may reflect only 1.5% of the light. An uncoated lens surface may reflect 4% of the light. Considering a binocular might have 12 to 18 surfaces, the importance of multicoating soon becomes evident.

Probably just as important is simply having multicaotings is the precise control of the thickness of the coating process. An improperly controlled multi-coating process can produce a lens coating that reflects more light than a single coating. Although you can't truely determine anything about the precision of the coating application by the color of the coating, in general, poorly coated lenses tend to look bright green. The bright green color could result from coatings being applied too thick. Improper coating thickness will reflect more light than it should, reduce the light transmission of the optics and if bad enough may cause internal reflections.

Lenses that fail the extremely tight tollerances required in the coating process should not be categorized as premium multicoated lenses. But they are still multi-coated lenses and will be used in the manufacture of some brand of binocular (same as objectives mentioned above that don't pass QC criteria). So you could be buying a fully multi-coated binocular that has lenses that don't meet stringent coating thickness criteria.

Here is a quote from a highly respected individual that has been associated with the optics industry. His comments below pertain also to my comments above re: QC of lenses and coatings.

Quote:

I've worked in the optical industry a bit, so I have a fair idea what the limiting factor in the quality of binoculars made for the mass market is.

Prisms aren't particularly easy to fabricate to really high levels of quality. The "quality sieve" method may help keep costs down, but they're still not cheap to make. (The "quality sieve" method of production was described to me by a fellow who had worked with the Japanese exporter of fine optics many years ago. A fabricator would make, say 1000 prisms, lens sets, whatever. Only the ones which met the higherst standards would be shipped to the most discriminating customer, say the best 200. The next 200 would be shipped to a customer who had high standards, but not so high as the "top dog", and so on, until the glass is all gone. The prisms that make it into the "no-name" binocs you buy at K-Mart for $19.95 likely came from the "bottom 200"!)

Rick Shaffer




So there you have it. They may look exactly the same on the outside, but Oh, there are so many possiblities for differences.

edz


These Look The Same, are they?
This post summarizes a lot of the quality differences that might exist be between various brands and price levels of binoculars, most of which would never be evident by looking at the outside of the binocular. With this post we try to do our best to provide you the information you need to dispense with the often heard internet myth that they are all the same. It's just not so.




Is it worth it to pay more to get more? Are you actually getting more? What are the problems with lesser priced products?


Things to watch out for in cheap binoculars

There are many other posts that discuss much more. I will link to them as I find them

Are these the same?



http://www.binocularschina.com/binoculars/giant_binoculars.html

http://www.united-optics.com/index.html


and last, but not least, this particular thread is an approach to this subject from a completely different angle
10 Reasons to Spend $1000 on "Little" Binos

edz

Edited by Zdee (06/27/12 08:06 PM)


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Cleaning Lenses - Binoc Repair Eyeguards - Caps new [Re: EdZ]
      #71869 - 03/15/04 09:36 AM

This is another question that keeps coming up all the time.

Here's some info. Note it varies by individual. Which method is best? The one that takes the most care with the least frequency. Cleaning binoculars is the same as eyepieces and scope lenses. they are all coated optics.

Caution should be exercised by any individual who happens upon this thread looking for information intending to clean a dielectric coated surface. Dielectric coatings include TV everbright diagonals and similar products

for binoculars and other normally coated lenses this offsite website provides excellent advice
Arkansas Sky Observatory - Fine Optics CLEANING SYSTEM

This is how an experienced optical repairman cleans
Cleaning Binoculars

Read this thread for cleaning methods by some of our users
More comments on cleaning

read what Orion says about how to clean binoculars
Orion website instructions for cleaning the Ultraviews

here's a link to an article over on the CN reviews side
Cleaning Eyepieces

here's what NOT to do
I ruined an eyepiece

Steps to take to help avoid cleaning
A quick, successful dust removal!

If you want to take the time to wade through this thread, there are plenty more opinions
cleaning EPs



Need Serious Internal Cleaning??
See this thread for links to repair services

Repair Services
links to binocular repair and/or collimation services
Binos need Cleaning or Repair or Adjustment about a dozen different links


Replacing Rubber Eyeguards and binoculars objective lens caps

Eyepiece caps and Lens caps
Binoculars and Eyepiece Lens Caps




Edited by csa/montana (10/08/09 09:44 AM)


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Need Binocular COLLIMATING or Repair? new [Re: EdZ]
      #73181 - 03/17/04 12:08 PM

For most binoculars with a minor need for an alignment adjustment, turning the prism tilt screws is the primary means available to the end user and is an appropriate adjustment, called conditional alignment, but commonly referred to as collimation. True collimation can only be accomplished by a qualified repair service with the proper instruments.

Repair Services
links to binocular repair and/or collimation services
Binos need Cleaning or Repair or Adjustment about a dozen different links
Suggestions Please / Repairs
Binocular Repair


COLLIMATION
Collimating binoculars takes on two different meanings. The first and most common that everybody knows is to get the images from one barrel to merge with the other. But the second and much more difficult problem is the alignment of the optical elements along the optical axis in each tube separately. The first is really conditional alignment while the second is true collimation.

See this article in CN reviews dealing with conditional alignment
“Collimating” Binoculars

And see this article here in the forum dealing with collimation
Binocular Collimation and conditional alignment by Bill Cook
2) Alignment that is within the original JTII standards can be considered to function as “perfect” in that the eyes can easily accommodate the error without introducing noticeable eyestrain. For example, an 8 power instrument should have errors equal to or less than 4’ Step, 10’ Divergence and 6’ convergence, with a 16 power having 2’, 6’ and 3’ respectively.

or
JTII @ 4.5 power
8 ' and under for step (dipvergence)
20 ' and under for divergence
15 ' and under for convergence

Cheers,
Bill


and see this for some ALLOWABLE TOLLERANCES
Test for Convergence or Divergence

also see this
A lengthy discussion of collimation tollerances
as posted on different websites and by different manufacturers.
You'd never know by the thread topic that this is 90% of the content in this thread.


MERGING THE IMAGES
I'd recommend you assess the degree of miscollimation before attempting any adjustments.

Do you normally wear glasses? Do you have astigmatism? If no, OK, proceed. If you do have astigmatism, do not attempt to even visually test for miscollimation without your glasses.

Are you familiar with some double stars, test collimation on a double star of known separation. This gives you a benchmark separation from which you can determine the severity of miscollimation.

There are allowable tollerances. Standards seem to be more than I personally can tollerate, but you could easily have 2 to 3 arcmin of miscollimation and need no adjustment at all. Standards would indicate greater deviation is allowable. The tollerances get much tighter as magnification increases. A 7x or 8x binocular with 2 to 3 arcmin miscollimation won't look bad at all. However, in a 25x binocular with even 50 to 60 arcseconds of miscollimation it may be difficult to merge images.

An excellent double to test collimation is Nu (v) Draco. It has a separation of 62". It's like having a 1 arcmin measuring device in your image. If your low power binocular collimation is within 2 or 3 times the separation of Nu Draco, you may want to just leave it alone.

Miscollimation would produce a well lit exit pupil with black edges all around. It may or may not be round, ...but it should have dark all the way around it.

If it is determined as necessary to adjust the binocs, the amount of turn on these screws is very very small. You might turn each screw less than a quarter of a turn to reach adjustment.

Keep in mind that adjusting the screws to merge the images is being done without the use of any lab equipment. The eyes can tell when images are not merged, but they cannot tell which of the four prisms are the ones causing the misalignment.

The general trend is to stick with adjusting the two screws closest to the eyepiece end of the tube. However you have no way of knowing that they are the proper screws to adjust.

Sometimes you see or hear discussion of problems related to binoculars that seem to be in alignment (merged), but the exit pupils are not round. One possible cause of this condition is adjusting the wrong screws. If the front prism is slightly tilted and is causing misalignment but the back prism is adjusted to reset alignment, what you now have is two prisms that are both set incorrectly. The result may be an image that appears centered and aligned with the opposite barrel, but it would not be surprising to see oval exit pupils, out of round diffraction pattern or field of view not matching the other barrel.

In some cases turning prism screws can make the situation worse

A root cause of this condition is an objective lens that is not seated exactly centered and perpendicular to the optical axis. This may be the underlying condition that is forcing the adjustment of the prism screws to get the images merged.

In a truely collimated and aligned binocular, prism surfaces are perfectly perpendicular to the incoming light path and prism pairs are also perfectly parallel to each other. Tilting prisms to accomodate any deviation of either of these above conditions to result in merged images is done to compensate for some other misalignment in the light path and may result in one of the exit pupil or field of view conditions mentioned above.

For any binocular with a very minor need for adjustment, turning the prism tilt screws is the primary means available to the end user and is an appropriate adjustment. However, True collimation can only be accomplished by a qualified repair service with the proper instruments.

Conditional alignment vs. true collimation

Visual alignment of the field of view can be checked in daylight. However it is not recommended that any sort of prism adjustment be made based on a daylight condition. For adjustment, the light source should produce such a small point of light, it is generally thought that stars are the best source for this visual test.

WHERE TO FIND THE PRISM ADJUSTMENT SCREWS
Back (Eyepiece end) prism screw
Front (Objective end) prism screw


MISALIGNMENT OF THE OPTICAL AXIS
If you adjusted the wrong prism or if you were seeing an out of alignment optical tube, similar to a refractor tube that is not collimated, you might see an oval exit pupil. You would not see an oval shape in the exit pupil from the objective end. It must be viewed in the exit pupil at the eyepiece end. From the objective end, you might see that it would force you to view off center to see the full circle of light rather than viewing down the center of the axis of the light path.

Look here
Cat's Eyes

Oval or cat's eye shape in the exit pupil would be a clear indication that the light path in that side of the binocular is not on the optical axis. Could be several things causing it. All have to do with the alignment of the optical path in that barrel. They all end in the result of reducing the light output, which can be significant.

Here is an excellent pictoral of the view thru one barrel of a binocular with interchangable eyepieces. in place of the eyepiece is a Cheshire eyepiece. You can easily see when the optical alignment is off in one barrel.
Using a Cheshire Eyepiece to check an interchangable eyepiece binocular


Generally there are three collimation errors to be concerned about. For all misalignments the allowable error is smaller as magnification increases. Actually this results in the same apparent visual error, the angular error multiplied by the magnification.

Vertical Alignment, a serious error, is when one image is higher than the other image. The eyes have no muscles to accommodate for vertical error. The allowable divergence is only 4 arcmin at 7x to 10x, only 3 arcmin at 12x to 15x and 2 arcminutes at 15x to 20x. Personally, I cannot tollerate even 2 to 3 arcminutes of vertical error even at magnifications of 10x and only 1 arcmin at 16x.

Horizontal Convergence, the least problematic error, is when the image in the right eyepiece is to the left of the image in the left eyepiece. You might think of this as cross-eyed. The most separation can be tolerated in horizontal convergence, 10 arcmin at 7x to 10x, 8 arcmin at 12x to 15x and 6 arcmin at 15x to 20x. Most eyes can readily accommodate this error, but these limits I find a little too wide. I could not tollerate even half that much.

Horizontal Divergence, another serious error, is when the image in the right eyepiece is to the right of the image in the left eyepiece. The images are spread apart, an error that the eyes cannot accommodate without strain. The allowable error for horizontal divergence is 6 arcmin at 7x to 10x and only 4 arcmin at 12x to 15x and 3 arcmin at 15x to 20x.

Keep in mind, the magnification of your binocular has a significant influence on how you see this error. Your eyes see the apparent error, or the angular error multiplied by the magnification. For instance a 3 arcminute error in a 10x binocular will appear to have a magnified apparent size of 30 arcminutes. That same error in a 15x binocular will have an unacceptable apparent size of 45 arcminutes and in a 20x binocular will appear as 60 arcminutes apparent error.

For reference 1 inch at 100 yards is just slightly less than 1 arcminute.

Personally I cannot accomodate my eyes to these tollerances. I would recommend cutting all of these in half. HD is the error I see almost all the time. Because of the way the prisms rotate, there is always some vertical alignment error with it. I can accomodate 2-3 arcmin error at 10x-12x. I begin to find it difficult to accomodate a 2 arcmin error at 15x-16x. I could not tolerate 2 arcmin error at 20x. In large binoculars at 25x or higher, I find any collimation error of more than 1 arcminute unacceptable. I have my BT100 at 44x merged to about 20 arcsec.

Rotational Error, a fourth collimation error, is when one image is rotated in relation to the other image. I have never seen it in any binocular.

Suspect your binoculars are out of collimation and you want to know by how much? Observe a known double star such as nu Draco (62 arcseconds) and allow your eyes to relax so you can observe the error. Use the spread of the double to compare to the spread of the collimation error.

Collimation error can be measured in an instrument by observing a variety of double stars of varying separations and watching as the stars try to merge. The separation of the double is used as a scale in your view. Compare the miscollimation error to the separation of the double. I test mid-sized 10x-15x binoculars using a 60" double. For 20x-25x, and for binocular telescopes depending on the power at which you use them, you need to test with doubles more like 20" or 15" or 10" or even less. To observe comfortably at powers between 50x-100x, you might need your BT merged to within a tollerance of less than 30 arcseconds.

A few test targets
Stf 485, Cam 7.0-7.1/18”, 4h05m +61n, easy, in o.c. 1502
Stf 2690 Del 7.0-7.2/16.7", 20h35m +11.5n,
100 Herc, 5.9-6.0/14.2"
5 Ari, gamma 4.8-4.8/7.8" PA 000,
11 Mon, beta, 4.7-5.2/7.3”, C=6.1/10”, BC=2.8”, seen as A-BC = 4.7-4.8/7.3”
Stf 232, Tri, 7.9-7.8/6.6”, 2h15m +31n, near 6 Tri,
95 Her, 5.0-5.1/6.3”, 5° ssw of 100Her,
16 Zeta Cnc, AB-C 5.0-6.0/6.0”,

edz



Collimation on a MkV explained by daniel_h who attended a course on binocular collimation taught by Cory Suddarth.

Does anyone know if it is possible to build/buy a binocular collimator for something less than $100?

A comment on collimating binoculars (or how you might screw up the collimation of your binoculars)

Where to find the prism adjustment screws on the Oberwerk Ultra, GO Signature, AP Premium, Resolux, Etc
Photo of Screw Location


Edited by Richard McC (11/24/11 04:28 AM)


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Contrast new [Re: EdZ]
      #75304 - 03/20/04 08:48 PM

In essence, adding magnification decreases exit pupil and reduces brightness, but it does not increase contrast. It only gives the perception of increased contrast on stars because the faint stars in the view becomes easier to see with added magnification. What happens when you add magnification to your existing aperture is the sky background, which is an extended object, gets darkened. But all other extended objects would get equally darker. However, the object has some perceptible light gradient difference (contrast) with the background and its contrast difference with the sky background is maintained, not increased or decreased. As the object gets larger the perception is it has become easier to see. While it may possibly be made easier to see with the optimum magnification, this is improperly interpreted as increased contrast.

Post in the above noted thread that includes links to some of the best written works on contrast.

Thread on CONTRAST with links explaining Optimum Detection Magnification


And posts in this thread that include even more works on ODM and contrast
Exit Pupil Overrated


Please refer to these two written works by Bill Ferris which provide some of the best and easiest to understand explanations of contrast that you are likely to find anywhere.
An Explanation of Contrast Ratio by Bill Ferris

An offsite paper by Bill Ferris explaining contrast threshold
Lowering the Threshold



What other quality aspects affect contrast?
Among other things, contrast is reduced by reflected internal light (which also is a result of poor or no coatings), improper or no baffles which permits stray light to interfer, poorly polished glass surfaces which produce scatter, poorly figured lenses which produce poor resolution and reduce the amount of light in the Airy disk, chromatic aberration which reduces the concentration of all colors of light in the Airy disk and spherical aberration which increases the size of the point image instead of concentrating the light to a point.

Are coatings more important for large exit pupils?
this thread title belies the content of the thread. The discussion revolves around contrast, but rather than just discussing the importance of coatings, it contains discussion of how contrast is not only affected by lesser quality coatings, but also by: sky conditions bright or dark therefore apparent contrast, larger aperture therefore larger exit pupil, smaller aperture therefore lower light gathering, lens aberrations, eye pupils, masking the aperture, lower magnification therefore larger exit pupil, higher magnification therefore smaller exit pupil but also larger image scale.



Users comment on contrast.

Please note, some of these following explanations apply to stars and this is improperly described as contrast. In some cases below this is explained.

Exit pupil is also a factor in contrast. I find that 3-4mm works best for my skies (also good for compensating for my astigmatism), but at 5mm the contrast starts to fade, at 6mm the skies begin to wash out and fainter stars disappear, and at 7mm, the skies look brighter through the bins than naked eye, if that's possible.

Performance of 8.5x44 vs 9x63 vs 10x50


A higher contrast, sharper image from a smaller exit pupil can exceed an image from a larger exit pupil if it's not from an equally high quality binocular.

What do you give up to get 7mm exit pupils? Image size. But if the image is broad and diffuse, do you need a larger image scale? Maybe not.

The binocular that would be best for broad diffuse extended objects is not the same binocular that would be best for most other objects. For most everything else, a higher magnification (that produces a smaller exit pupil) provides a larger image and a bit deeper limiting magnitude that allows seeing more.

Think of the sky as the backdrop in a portrait. Your subject must stand out from the backdrop to be seen. (There must be contrast.) Consider it to be a grey scale. You all understand the affect of dark sky background. Higher magnifications in a telescope reduce the extended light of the background until it approaches a completely dark sky, to the point you can no longer see the field stop. Lower magnification with a larger exit pupil delivers more light, not only from your subject, but also from the background. This can be beneficial or detrimental, depends on the condition. Generally, this makes it much more difficult to see most objects, but in dark conditions may allow you to see nebulous objects.

Exit Pupil, Large or Small


There is no question that a larger aperture gathers more light. But it gathers not only the point source light, but also the sky background light. Under a dark sky, with far less sky background light, the performance curves are much further apart. Under a bright sky, the performance curves are almost equal.
But now on to (C), the most significant point Fiske highlights here. Let me repeat it.
(C)They (comparisons of performance as determined by point source LM) do not reliably represent performance on dim, extended objects. Try doing the 10x50mm versus 10x70mm comparison on a faint open cluster like NGC 7789 in Cassiopiea and see which instrument comes out on top.

This point cannot go overlooked. The difficulty is now involved in assessing which is better when viewing extended objects. This can be a subjective call. Point source LM is not subjective at all. However, it is not a good indicator of performance of contrast on extended objects.

There are a wide ranging class of objects that would fit the criteria "extended" in binoculars. Of course, all the typical extended nebula (Neb) and galaxies (Glx), but also some open clusters (OC). OC, if very dense may be resolved in scopes but may not be resolved in binoculars, and especially if comprised of predominantly faint stars. NGC7789 probably fits this criteria pretty well.

The difficulty comes in objectively assessing the performance on objects where you don't get a maximum or a count or something objective. This doesn't make it impossible to judge the differences in contrast, just more difficult.


Comment about Exit Pupil Size


"Contrast Enhancement via Magnification--As the magnifying power of an eyepiece increases, the amount of light reaching the eye decreases. However, a modest increase in magnification is often found to enhance the contrast between stars and the surrounding sky, and this effect can sometimes be exploited when making estimates of relative magnitude in moderately light polluted skies. It is frequently found, for example, that 10x-50mm binoculars are preferable to 7x-50mm binoculars in less than totally dark skies. The same holds true for a telescope, and you may find that an increase from a low power to a medium power eyepiece, say, from 20x to 40x, will provide a more favorable viewing situation under marginal conditions."
-- Sommers-Bausch Observatory -- University of Colorado:

"The glow of the background sky is a diffuse source, which will be spread out by higher magnification, reducing its brightness. Although high magnification doesn't increase the brightness of faint stars, it improves their contrast (ed. note apparent brightness, this article inproperly refers to this as contrast) against the sky, making them more visible!"

On the night sky, apparent contrast has *EVERYTHING* to do with magnification, light pollution, seeing conditions, surface light scatter, internal light scatter, quality & thoroughness of coatings, baffling, quality of optics, etc. In short, perceived contrast is a direct result of the summation of the whole instrument, the observing site and conditions and the observer.


edz






Edited by EdZ (02/13/10 07:15 AM)


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EdZ
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Reged: 02/15/02

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MEASURING / TESTING Optical Aspects of Binoculars new [Re: EdZ]
      #75703 - 03/21/04 03:30 PM

This first link is to one post that gathered a bunch of links as help to others who want to review the types of tests they can do to measure the performance of their binoculars. So go to this post and you will find about a dozen links to other posts, mostly here in the "Best Of" threads, on how to measure various aspects of binoculars.
Links for Aspects to measure as posted in the thread "Writing a Binocular Review"


Calculating TFOV

EdZ's list of sky distances for measuring True Field of View in binoculars
How To Measure TFOV Of Binoculars

Image Sharpness Across the Field of View
Another more 'full length' discussion of Filed Sharpness discussing a method how to measure accurately so binoculars can be compared.

Measuring True Sharp Field of View



Measuring the Magnification of your binoculars
Measuring the size of an exit pupil

Measuring magnification by observing a standard scale

Measuring Aperture by three methods
observing a scale in the exit pupil directly using a loupe
checking magnification then checking exit pupil
measuring projected image of a leaser thru the objective

this thread above has a great deal of information. For instance if you want to check aperture by measuring exit pupil, you must first verify the magnification. There are several methods to check magnification. They are also discussed in this thread above.

True aperture? A Quick Way to Measure!
Measuring effective aperture by the flashlight method. Simple and effective.
Flashlight Aperture Test Improved


and more on
Measuring Aperture, Illumination, Beam Tilt, Collimation
EdZ's 22 x 85 GO Signatures
This post contains essays explaining each of these tests outlined below
Checking aperture by using masks and checking exit pupil
Mask method explained in detail
Checking Illumination of the Exit Pupil (Beam Transfer) with a target laser
laser method explained with pictoral presentation
Results compared with a dozen other binoculars
Checking Optical Path Tilt with target laser
Using the Target Laser to guide adjustment of conditional alignment
The pitfalls of conditional alignment when some unknown cause of tilt is present.
A perfectly balanced binocular, The Fujinon FMT-SX 10x50 has full aperture, wide area of illumination, No optical tilt, and good alignment.


How to Measure Magnification and Aperture using masks
an example of the measurements that shows how this simple method works

Things to Watch Out for when measuring magnification

Excerpts from posts on explanations of how to measure magnification:

For magnification, see Henry Link's reference to Kimmo Absetz’s method of establishing a reading for a small power monocular:
Take a reading of a tape measure at some distance (75'to 100' away) using a monocular, then take a reading of the tape thru the binocular with the monocular behind the binocular eyelens. [Used a small scope to accurately get a measure on a scale about 100 feet away (reading approx 14-15 feet). Then used the small scope behind each binocular to take another reading and divided the two values.] Division of the readings gives the power. All magnifications reported here use that method. It gives a fairly accurate reading, but because of it's relatively close (100feet in my case) distance, all readings may produce magnification that is slightly (approx 1%) high. (In any binocular, Magnification varies usually by ~2% and by as much as 3%, from closest focus to infinity). However, anyone trying this method would see that it can be quite accurate and it is easy to measure power to tenths. Therefore, results are reported to 1/10th. All actual calculations produced values to hundreths, but there is no way that I can be measuring with that level of accuracy, so I have not reported it. It is impractical to measure using this method much beyond 150 feet and in fact impossible to test using this method at infinity.

OR

Place one eye to the binocular viewing a one inch blocked out (using 3M stickies) area on a scale and one eye looking at the same scale marked off in large blocks of inches, ie.' 7", 8", 10". Superimpose the images of the magnified 1 inch block over the scaled blocks (powers) of 7 or 8 or 10, whichever fits. It becomes easy to see them line up. If the magnified 1 inch block lines up with the 8" scaled block, it's 8x. I find it particularly easy when the magnified image falls in the middle of some colored sticky, showing for instance 8.5x. I can see powers down to an accuracy of about a quarter inch or 0.25x. Problem with this method is it needs to be done only at close distance, perhaps 20 feet to 50 feet at most. All binoculars increase in magnification at closer focus, so the result does not represent magnification at infinity, it represents magnification at some closer focus distance. But, since that close/infinity variation is perhaps only 2% to 3%, errors larger than that can be found easily. It takes some practice to let your eyes view two completely different images and superimpose them. I find it pretty easy to do.


OR

Measure the exit pupil as is at apparent full apereture. Record.

Assuming an 80mm binocular)
Then place a lens cap that has been cut to a 70mm aperture.
Measure the exit pupil WITH the 70mm mask. record
Then place a lens cap that has been cut to a 60mm aperture.
Measure the exit pupil WITH the 60mm mask. record
If I were using this method to test 50mm binoculars, I would test at 50mm, 45mm and 40mm.

You must measure exit pupil precisely to at least 5/100ths mm, so you need a precise dial or digital caliper. You would not be able to measure this precise with a $10 vernier caliper.
You must measure the mask precisely to at least one half mm.

Do the math for each aperture. The masks will show the point beyond which you have covered up all internal vignette and eliminate vignette from the equation. You need at least two measurements to agree.





How To Measure the Focal Length of Binoculars
Formulas you need to know and assumptions you can make if you want to estimate the focal length of your binoculars.
You must take into consideration the refractive index of the prisms.



Here's a simple test to compare Coatings from one model to another
Look for reflections in the coatings

See the "Best Of" post on coatings for much more.


Testing Edge Sharpness by observing double stars with known separations
Oberwerk BT100 - Stock 24.5mm ep sharpness
and Barry Simon tells us how he tests for edge distortions and Field of View in
Pop quiz: 22x100 Obe's or 20x77 Miyauchis



Deciding when it is necessary to adjust the collimation
Collimation

Measuring Eye Relief
and determining the difference between measured eye relief and usable eye relief


Exit Pupil, Brightness and AFOV
Does AFOV affect the brightness of the image in the exit Pupil?
How to test the Illumination of the Exit Pupil

Brightness in the Exit Pupil

and please excuse that I borrow this from another forum but here is an excellent discussion on
Thoughts on exit pupil, brightness, and resolution
it actually discusses the measuring of brightness in the exit pupil


Edited by Richard McC (03/22/12 06:17 AM)


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