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True aperture? A Quick Way to Measure!

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#151 Erik D

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Posted 06 January 2010 - 09:43 AM

[quote name="F.Meiresonne"]I don't have as much confidence doing the measurements myself.[/quote]
neither have I. Did does not seem an adequate method... [/quote]

Freddy, Todd,

To clarify, I WAS able to achieve repeatability time and again with assistance. My lack of confidence is if I am holding the flashlight myself and trying to measure with dial caliper at the same time.

As doctordubs photo shows, it was relatively easy to project a sharp disc with a red LED light. With two helpers'eyeballing & guiding me I was able to position the caliper just so for precise measurement.

When two of them agree I was holding the caliper arms square and tangent to the disc I can lock the caliper and get a reading to .001 mm. Repeatability to within 0.5 mm was easy.

I have dozens of spare tripods and heads in the house. If I can rig up something to hold a red LED light I think I shall be able to get repeatable measurements myself. My precision steel rulers have scale to 1/64 inch. I'll have to find a mm ruler.

It's one way to achieve repeatable readings of aperture, but other data such as true X and % illumination require someone with the skill and patience of Prof Ed. I know there is no way I will attempt to estimate X by comparing a magnified image to true image myself.

Knowing true aperture is just one factor for predicting binocular performance. It's good to be able to verify factory specs for astro users but it's not of that much concern to most terrestrial observers.



ERik D

#152 Tony Flanders

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Posted 06 January 2010 - 11:23 AM

I WAS able to achieve repeatability time and again with assistance.


For what it's worth, repeatability is *not* a guarantee of accuracy. It's true that lack of repeatability is a sure sign of inaccuracy, but the converse is not true. Repeatability often results from systematic error. The history of science is replete with examples.

I only really trust measurements when they're done completely independently by different people using different methods.

#153 EdZ

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Posted 06 January 2010 - 11:26 AM

Geez, this method is a lot simpler than this.

I managed to get good results by meself, holding a white flashlight behind the eyepieces, moving it around and watching the light projected onto a clear plastic ruler. results looked accurate to within a mm.

This method does not require tripods, several people or calipers.

edz

#154 richtea

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Posted 06 January 2010 - 11:52 AM

Hi

Actually i have to say Edz's method is exactly the one i used and this came up quite quickly with the results being close to a mm or so on all the bins i tried
Believe me i am far from technical optically so on this basis try the simple flashlight and ruler version
Have a little tipple or drink to relax maybe first (wine often helps me !!)
In honesty i found it worked even when the flashlight was not entirely centred bang on
If it was pretty close to centre it seemed ok
Its a really handy little measuring thing for those who want to know if their bins are slightly restricted in specificied aperture

Regards
RichT

#155 F.Meiresonne

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Posted 06 January 2010 - 04:15 PM

Geez, this method is a lot simpler than this.

I managed to get good results by meself, holding a white flashlight behind the eyepieces, moving it around and watching the light projected onto a clear plastic ruler. results looked accurate to within a mm.

This method does not require tripods, several people or calipers.

edz


I can't get a 'clear' edge. I've tried it with my Obie Mariner 10x60. Wel it sure is under 60 mm but it's 'something' between well 53-57 mm.
Seems to me a rough method :confused:
I used red light.
Guess i am not good for the measuring business. Maybe i should stick to look 'trough' them in stead of 'at' them.

#156 Mateyhv

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Posted 06 January 2010 - 07:33 PM

Freddy, try putting a strong flashlight at least 50cm from the eyepiece, at least it worked for me. Projecting the light coming from the lens on a wall from a close distance is Ok. 57mm may be a good reading. If edges are not clear maybe there are some internal diffraction.

Matey

#157 Tony Flanders

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Posted 07 January 2010 - 08:50 AM

I managed to get good results by meself, holding a white flashlight behind the eyepieces, moving it around and watching the light projected onto a clear plastic ruler. results looked accurate to within a mm.


Moving the light around introduces a potential source of error -- though the error will be small if the ruler is directly in front of the objective.

I must say, though, that this test seems *very* easy to do -- like a minute to plan and 10 seconds to execute -- as long as you're willing to accept that it might be off by a mm or two. Which is a pretty tiny amount in practice.

It helps greatly to have a flashlight with a small, intense beam. My white LED flashlight seems like just the ticket.

#158 Erik D

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Posted 08 January 2010 - 05:48 PM

I purchased a pair of classic Japanese 20X80 from Orion in late Oct 2001. The Orion holiday 2001 catalog also listed a 11X80 & 16X80 model at the time.($399 & $409) EdZ reviewed the 16X80 vs. a pair of Oberwerk 15X70s some years back.

As far as I know my Orion 20X80s, priced at $419 a pair, were near the upper price range for 20X80s back then. Only the Orion MegaView, with integral, weight balancing 1/4-20 mounting post cost more. Price was $489 a pair for the MegaView 20X80. ($479/15X, $499/30X)

I selected the Orion 20X80s because the hinged metal bridge connecting the objectives. The Orion's catalog also said "Each binocular is tested prior to sale on a laser collimator to insure proper optical alignment". Gave me a sense of confidence about my very first pair of 80mm binos I have wanted since the early 1980s. My Orion 20X80s are identical to the classic Clestron 20X80s except for the Orange colored Celestron trim ring and logo on the carrying case. The Celestron 20X80 are discussed in this current thread:

http://www.cloudynig...5/o/all/fpart/1

The Orion 20X80s did not disappoint when they arrived. Orion doubled boxed them. The inner box was cushioned by a layer of foam peanuts. IIRC, the binos were bubble wrapped inside the carrying case. 3 layers of protection! The built quality is SOLID. Just like my Celestron Nova 7X50s from the mid 1980s.

Earlier this week I took my Burgess 20X80 LWs to the office to check clear aperture with my friends assistance. My earlier test of the Leupold 12X50s encouraged me. As I mentioned in a previous post. I found that it was possible to project sharp disc using my friends RED LED flashlight. The 20X80 LWs measured ~75 mm in each barrel. It was easier to define a sharp red disc from the right barrel. Had to move the light around a bit to get a sharp disc from the left.

Yesterday I took my Japanese Orion 20X80s to the office to try the same. Had a little more trouble getting a sharp ROUND disc compared to the Chinese 20X80 LW. More so from the right barrel.

After moving the light around a bit I measured the left aperture at ~73.5 mm. Noticed a very small obstruction shadow near the edge, at the 2 O'clock position. I would say may 1/2 the size of the nail on the pinkie of a little girl. Had more trouble with the right barrel. Had to tilt and move the LED light several times more to get a round disc. The edge of the red disc didn't seem as clearly defined as the one from the left. Noticed the same small dark shadow also. I would guess it's less than 5% by area.

So here is the result of classic Japanese 20X80 ($419) vs Chinese 20X80 LW ($149): 73.5 mm left/74 mm right vs. 75 mm left/ 75 mm right.

I wish I had an instrument to measure the intensity of the light disc after it had passed thru the optical train. To my eyes I can not tell if one pair is superior to the next on the night sky. I use the 20X80 LWs a lOT more often because they seem to have a wider Tfov/Afov. Weight is nearly two pounds lighter. The Orion 20X80s have been in the storage case for about two years. I am keeping mine because I don't need to sell my near-mint Japanese 20X80s for $120. ;-))

Any one checking out these two pairs side by side in a retail store will likely select the Orion as the binocular with higher quality optics because of the superior mechanical construction, .

--------------------

My friend"s LED light also has a white light mode but it's not usable for this test. It has 4 or five elements. Impossible to focus to a sharp disc.

My only readily available light sources at home are 2 AA and 2 AAA Maglights with adjustable focus beam. It's impossible to define a sharp disc with conventional old fashioned Maglights no matter how I move, tilt, forward and back with the light. I have not taken measurements at home. It's pointless to try unless I have a better defined light source. My results would be similar to Freddy's....maybe 71 mm to 75 mm. Not very useful at all.

ERik D

#159 KennyJ

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Posted 08 January 2010 - 06:16 PM

Erik ,

It would appear from your much appreciated detailed test reports that the exit - pupils from both of your 20 x 80 binoculars are closer to 3.7mm than the 4mm one would expect from a true 20 x 80 instrument .

Knowing you have not only at least 20 different binoculars at your disposal , but a lot of experience of actually LOOKING THROUGH many of them , I was wondering if you could utilise a combination of both to assess and reprt back as to whether or not you would estimate the exit pupils of your 80mm binoculars to be less than 4mm , or the TRUE magnification to be less than the stated 20x , or perhaps an element of both ?

Kenny

#160 EdZ

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Posted 08 January 2010 - 06:57 PM

I owned a Burgess 20x80 LW for about a year. Sold them about a year or two ago. I measured tham at 70mm, the smallest of all the 80mm binoculars I've tested. I measured the magnification at 18x.

The small exit pupil one might expect from a binocular with reduced aperture often times does not materialize because not only is aperture reduced but magnification is lower than specified, keeping exit pupil closer to what one might expect from full specs.

edz

#161 Erik D

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Posted 09 January 2010 - 12:51 AM

repeatability is *not* a guarantee of accuracy. It's true that lack of repeatability is a sure sign of inaccuracy, but the converse is not true. Repeatability often results from systematic error.


Tony posted the above shortly after I reported the aperture measurement of my 20X80 LW and I totally agree with it.

I was very careful in choosing my words to report my confidence of "repeatability" with a particular RED LED light source and measuring device on a given day, not in the absolute accuracy of my measurements. Both barrels of the 80 mm LW appear to be ~ 75 mm +/- 0.5 mm.

No way of knowing if I shipped the same pair to another member, with a white light source, a clear plastic ruler, just as confident about his repeatability, will result in the same measurements. Or if we repeat the same test on a different day, with my friend holding the caliper and I the LED light will have the same measurement.

As a former engineer I like to remove the variability caused the operator as much as possible in any measurement exercise.

KennyJ, You have more confidence in me than I have myself. I just reported the efforts I went thru to measure a sharply defined 75 mm red disc on a wall. Now you want me to try it with 3.75 mm virtual image?

Let me remind you my +/- 0.5 mm confidence window will result in ~13% error in exit pupil measurement for a pair of 18X70 mm binocular ;-))

ERik D

#162 EdZ

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Posted 09 January 2010 - 07:44 AM

Tony also made another comment in that same post, something on the order of, it's good if one method of measure can be verified by some other method of measure.

Long before this thread started describing a quick and simple method of aperture measurement, I have documented in this forum four other methods by which an accurate measure of aperture can be found. For practically every binocular I've reported on in this forum, I've measured effective apperture by no less than three different methods, some of them more accurate than this method outlined here.

I never got the opportunity to measure the Burgess using this simplified method (but that certainly is no loss as I've been using more accurate methods over the years). However, I did measure it three different ways, one of those being more precise than this method. All three measurements agree closely at approx 70mm. FWIW, I also had opportunity to measure the Barska XTrail 20x80, which is the same binocular as the Burgess. The Barska measured 18x72.

The Orion 16x80 that I tested 6-7 years ago was the Orion Giant 16x80. At that time it cost about $399 and was of Japanese origin. Looking back thru records, a test that I performed at that time (although back then I was not reporting on reduced apperture) seems to indicate that Orion Japanese 16x80 was actually 72mm aperture.

FWIW, I have data on 7 out of 8 of the 80mm binoculars that I've tested over the years. I haven't found a single one of them to have effective aperture greater than 72mm.

edz

#163 GlennLeDrew

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Posted 12 January 2010 - 08:32 PM

ERiK D,
Measuring something as small as an exit pupil is best done with the help of magnification. I have a 10X loupe with millimeter scale divided into tenths. 1/10mm at 10X is so well resolved that it's not at all difficult to measure to 0.05mm, or 10X better than with your caliper and unaided eye. So your error would be reduced accordingly, to of order 1.3% for your 3.75mm exit pupil.

#164 Man in a Tub

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Posted 13 January 2010 - 02:40 AM


FWIW, I have data on 7 out of 8 of the 80mm binoculars that I've tested over the years. I haven't found a single one of them to have effective aperture greater than 72mm.

edz


After today's "rigorous" effort, I'll say ~ (72 ± 1) mm for my Oberwerk 20x80 Standard. My previous results ranged from 70mm to as much as 75mm.

#165 F.Meiresonne

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Posted 13 January 2010 - 06:22 AM

No wonder my 'sold' Helios 20x80 could not see deeper then my TS Marine 15x70....
10% decrease in aperture as advertised. That's alot

#166 Erik D

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Posted 13 January 2010 - 07:29 AM

I am sure that on last Monday, January 4th, measuring the sharp disc projected by a red LED light on the wall with digital dial caliper, verified by two coworkers, the aperture of my 20X80 LW was MUCH closer to 75 mm than 70 mm, or even 72 mm. As I said, my confidence window that day was +/- 0.5 mm. Any bigger there would be a gap between the disk and the caliper fingers, smaller and we can see the red light overflow. We did not take different readings 90 degs around to verify if the disc was truly round.

I Only had one helper on Jan 7th when we measured the Japanese Orion 20X80, had more difficulty projecting a sharp red disc from the left barrel. Had to move the LED around quite a bit. But when we did there was little question it measured Smaller than the 20X80 LW's ~75 mm. There WAS a difference between the left and right barrel of the Japanese Orion. Very subtle but measureable 73.5 mm vs 74 mm. I understand others' measurements of Chinese 80 mms are closer to 70-72 mm. My Burgess 20X80 LW was purchased in March 2003.

Two things come to mind:

1. There are considerable sample to sample variation among low cost Chinese 80 mm binos, even if they are the same brand and model, OR

2. Our measurement results can vary depending on the set up and the person(s) performing the measurement, as Todd, Freddy and myself have tried to point out. Not as clear cut as others expressed.

Precision is not the same as accuracy.
-------------------------------

I have no way to cross check my measurements with laser projection at this time. I am not even going to attempt to estimate actual magnification of each pair. I do have magnifying loupe somewhere . I can try to reduce my error margin of exit pupil measurement as suggested by Glenn.

I guess one conclusion I can make, from my own comparison of one each of Japanese 20X80 vs Chinese 80 mm LW, is that this particular pair of LWs have slightly bigger true aperture, and no visible prism obstruction.

As usual, I am sharing my own findings, observations and subjective evaluation, not a scientific report.

It's nice to be able to go beyond the manufacture's listed specs on forum such as this. There is no better place on the net. But in the end, how the image appears in your own eyes, under YOUR particular observing conditions, is more important. For example, my own binocular use is mainly is the day time, not that I don't have a passion for astro viewing, but unless I have a dark, clear, moonless night, (rare for many of us) 5-7 mm aperture difference in a 80 mm bino is not going to matter much in my observing pleasure.

23X93 mm WP binos can be had for for less than a pair of Japanese 20X80 10 years ago, less than a single premium 100 deg EP today. No one says we have to stay with 18X72 mm binoculars only if we want to see dimmer objects.

ERik D

#167 Mark9473

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Posted 13 January 2010 - 03:33 PM

Happy birthday, Freddy!

#168 F.Meiresonne

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Posted 13 January 2010 - 06:28 PM

Thanks Mark

#169 GlennLeDrew

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Posted 15 January 2010 - 08:40 PM

For instruments suffering from aperture restriction due to a too-small front prism aperture, the position of the eyepiece can have an effect on the measured value of effective aperture. To see for yourself whether this is of concern, try this test with the eyepiece focused fully in and then fully out. You *may* be able to measure a difference in effective aperture, and here's why.

This test uses the eyepiece optics to form a small image of the light at (ideally) the same image surface produced by the objective. It's this virtual light which makes the test possible. But if the eyepiece is positioned incorrectly, the image of the light will fall either in front of or behind the objective's focal surface.

Let's examine the situation when the eyepice is focused too far out. In this case the image of the light will lie behind the objective's focal surface. That is, the image of the light is too far rearward (just as the eyepice itself is too far rearward.) With our virtual light now effectively located farther away from the prism assembly, the offending front prism aperture appears angularly smaller as 'seen' from the position of the virtual light. The prism aperture behaves as though it were a bit smaller. The result; a smaller effective aperture.

And it follows, then, that an eyepice focused too closely inward will result in the opposite. Namely, that the effective aperture will measure as larger than when in actual use.

So the moral is this. Focus the bino on a distant object first. And what matters is that the focus be set for YOUR eye.

#170 Astrojensen

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Posted 16 January 2010 - 04:09 PM

Hey, hey! This is a fun test! Works well, too.

I hadn't read half the entries in the thread before I tested my own binoculars, curious as to what I might find. Well, no surprises here. All average results.

First up was my old no-name japanese 10x50's from the 1970'ies. I think my dad got them for christmas in 1980. Using a Maglight, I measured them to actually be 46mm's, with some dimming visible from the Bk7 prisms. No surprises here. What scared me more was that looking backwards through the illuminated binoculars, I saw what appeared to be fog on the back side of the front right prism! I hope it's temporary.

Next on the table was my newer 8x40, which are still 12 years old now. These measured at 38mm, with some minor clipping of the illuminated disk, which was otherwise sharp and evenly illuminated.

My old 12x80 Tasco's proved to be another case of "not-quite-80mm" with results around 74-76mm. I didn't spend too much time on these. The illuminated disk also showed some light falloff due to the Bk7 prisms.

My 8x21 Optal's tested at exactly 20mm. Not too bad. Illumination was very even, except for a small butterfly-shaped brightening in the middle of the view. Could this be because they are roof-prism type? All the others are porro's. The butterfly shape was identical in both halves.

In other words, all results more or less as expected. None of my bino's are high-end, so it's no wonder they tested as they did.


Clear skies!

Thomas, Denmark

#171 Jay_Bird

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Posted 17 January 2010 - 05:17 PM

Here are some numbers using the ruler and loupe method:

Fuji 16x70: 69-70mm, nice round exit pupil

Comet King 11x80: 74-75mm, also round

Scope 7x50*: 46-47mm, round with less intrusive vignette**

Orion Scenix 7x50: 46mm, slight typical "10 and 2 o'clock" small chord prism cutoff otherwise round and bright

Bushnell Legend Porro 10x50: 46mm, slight prism cutoff (2 small chords), otherwise bright and round

SARD 6x42 wide: 41-42mm, nice bright round pupil no cutoff

Bell&Howell 8x40* wide: 38mm, round with less intrusive vignette

Wards 7x35* wide: 32mm, round, slightly more intrusive vignette

Swift 777 8x30 roof monocular/microscope: 27-28mm, round

AMC 7x35* 8° FOV: 34mm, round, less intrusive vignette

Manon 16x50* 4° FOV: 48mm, round, less intrusive vignette

Scope 7x35* wide: 31mm, most intrusive vignette and a prism cutoff (one larger chord instead of the usual 2 small chords)

* Indicates low index prisms. Some appear to have 'darker' or more intrusive **vignette than others; by vignette in this case I'm referring to the relative darkening in the exit pupil outside the square or diamond of full prism transmission. Is my perception wrong, or is the degree of shading outside of the low index 'brighter square inside a circle' an effect that varies in intensity, perhaps with objective f/ratio or prism size? Also, is some polarization introduced by the low index prism partial reflection?

#172 KennyJ

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Posted 17 January 2010 - 05:28 PM

It's seeming more and more to me like the quickest and easiest way to make an average approximation is that for ALL binoculars that either cost an arm and a leg or were made before 1960 , assume accurate specs. and for those priced suspiciously low , simply measure the diameter of one of the objective lenses and subtract 5% to 10% , depending upon how cheap they were to buy ! :-)

Kenny


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