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A Comprehensive Comparison of 4 70mm Binoculars



February, 2003

INDEX

Introduction
Index
Summary of Findings
Manufacturers Specifications - general info, selling points and price
Quality of Build features, hinges, focus, diopter, baffles, coatings, prism cut-off, collimation
Hand Held Impressions ergonomics, size, weight, fit, reach, eye relief, glasses, eyecup
Focal Length Fast achromats, how to measure
Importance of Magnification Mask test, vs. Aperture, magnitude, resolution, baseline index
Contrast and Resolution Resolving Fine Detail, Light Gradients, Binocular Performance Index
Object Finding - Maximum Field of View, examples of Max fov, field measures, AFOV
Types of Viewing finding limit, wide detail viewing, best detail viewing, on-axis
Sharpness Useable Field of View Data
Results of Sharpness Test Finding Limit, Detail Viewing Limit
Object Viewing wide detail viewing, best detail viewing, on-axis detail
Object Viewing Binocular Observing Report, viewing list, doubles in binoculars
Attributes- Brightness, Contrast, Resolution, Star Test, Color
Aberrations- Curvature (rectilinear distortion), Chromatic Aberration and Astigmatism
Credits

Update 8-17-03
Recently, I have received several emails from people that have read my Comprehensive 4way Feb 2003 article. They have questions about the newer version of the Oberwerk 15x70 with the upgraded coatings. Since I have two posts over on the forum side that address the issue of improved performance with these upgraded 15x70 binoculars, I would like to update this article with this brief paragraph and a link to these two posts. Thank you, edz.

Oberwerk 15x70 2002 vs 2003
Discussion #1

Rare night in New England! Oberwerk 15x70/03
Discussion #2

INTRODUCTION

In summer 2002, I published an article here on Cloudy Nights comparing 4 pair of binoculars; Orion Giant 16x80s, Oberwerk 15x70s, Orion Ultraview 10x50s and Minolta Standard XL 7x35s. It was a real learning experience because it forced me to discover a lot of things in order to compare all the aspects of different criteria. Since that time I ve bought more binoculars, returned some, and have found a few I really like. Some of you will only ever have a desire to own one pair of binoculars, maybe two. The choices are many and the more information you have the more informed choice you can make.

I have long been an advocate of binoculars for first timers because inexpensive binoculars will outperform inexpensive telescopes and they are far more likely to be used more often. I currently own eight pair. The biggest reason for owning all these is I like to set most of them up when I do outdoor programs for schools kids and scout troops. But at other times I get to compare them all and take notes. This article, like last summer s, makes objective comparisons, tabulates comparative data and notes conclusions and personal opinions of the binoculars I ve been using. I ll reiterate some of the criteria I set forth when I started my search. They must have at least a moderate field of view, I m looking for a good sharp image, they must be well within my price range and quality vs. price must be relatively high.

The binoculars I compare in this article are Pentax PCF 12x50, Oberwerk LER 15x70, Fujinon FMT-SX 16x70 and Oberwerk Giant 20x80. While this article will remain primarily a comparison of these makes and models, for certain criteria it will also reference some of the other binoculars I ve been using, including Minolta Activa 7x35, Nikon Sky & Earth Kestrel 10x50, Orion Ultraview 10x50 and Orion Giant 16x80. This should put performance in perspective and will allow some readers to get a feel for what can really be seen with these big binos as compared to smaller binoculars that many of you already own or desire to own. I find each of the above power/aperture ranges has it s own advantages. The 7x35s and 10x50s are great for hiking, bird watching and finding in the sky, the 70s and 80s are great for clusters, nebulae and can split double stars down to 10”. What I found is one binocular can t do it all. This article is approximately 30 pages long. You may want to approach it in sections.

Referencing the original article will provide you with a great deal more information laid out in the same format as this. The article is titled Oberwerk 15x70 vs. Orion Giant 16x80. http://www.cloudynights.com/breviews/4way.htm

Here in Cumberland, Rhode Island, U.S.A., 15miles north of the light dome of Providence, skies for our astronomy nights generally range between 4.0 and 5.0 mag. On good nights I can see the Little Dipper or all of the Circlet of Pisces except for TX Pis. I hope you find this article useful and informative. Clear skies, and if not, Cloudy Nights

SUMMARY CONCLUSIONS

I had been using the Oberwerk 20x80s for a while and was comparing them to the Oberwerk 15x70s. The 15x70s 4.3° fov allows sharp viewing of very wide star fields and larger nebula with a greater surrounding perspective than the 80s. They have a wide enough field of view to be used for finding objects. Image scale in the 80s and the depth the additional magnification and aperture provided is an impressive jump up from the 70s, but at a cost. The 80s are nearly twice the cost, significantly larger, require much more mount, have a narrower Tfov and do not operate quite as well mechanically.

I had always wanted a pair of Fujinons. So, when I had a chance to buy a pair of 16x70s at the right price I did, and I am convinced now that I will never regret that decision. The increased contrast and resolution in the Fujinon 16x70s provides the ability to see more stars than the 20x80s in every open or dense cluster. Also, faint diffuse objects appear more readily in the Fujinons than in any other binocular I own. The Fujinon see fainter objects and see more stars, but the 20x80s seem to have a slight edge when it comes to splitting close double stars.

The Pentax 12x50s measured a hair less than their stated 4.2° fov and are very sharp across 70% of the field, with a fairly sharp view all the way out to 90%+! These Pentax have a wow factor when you first see the image, it s that pinpoint and contrasty. The Pentax equal the best eye relief, 17mm, they have the best focus mechanism, and even though only 12x they saw almost every faint star seen in the 15x Oberwerks, although a very few were harder to see. I think better coatings and baffles in the 12x50s contributed to the ability to see almost the same as the 15x70s. They only weigh about as much as the Orion 10x50, so they can be hand braced.

The most versatile binocular for finding objects is the Orion Ultraview 10 x 50s (refer to the details in my previous binocular comparison). They have a wide enough 6.0° Max FOV to orient in perspective and the 10x magnification is satisfactory to see many faint objects. The 10 x 50s provide a comfortable rendition of sky as compared to the scale and magnitude limit of ‘SkyAtlas 2000.0 , reaching stars just beyond the atlas.

Once again, magnification turns out to contribute more to improved performance than aperture. The results of the aperture mask test show that as magnification increases, the utilization of the available detail provided by the aperture increases. For all aspects but brightness, aperture is under-utilized and performance increases as magnification increases. Better contrast and resolution noticeably improved the seeing ability of the binoculars. The Fujinon and the Pentax easily surpassed the respective Oberwerks in side-by-sides. The Fujinon did so by a wide margin. The two Oberwerks had the most internal reflections off the barrel and the most external reflections off the lenses of the dozen binoculars I ve tested so far.

Only the Orion 10x50s can be hand held. The Pentax 12x50s are marginally hand hold-able. The Oberwerk 15x70s at 15x are better off mounted. Both the Fujinon 16x70s and the Oberwerk 20x80s are very heavy binoculars and are not intended to be hand held.

The Fujinon 16x70s were by far the overall stellar performer in this grou

MANUFACTURERS SPECIFICATIONS AND MEASURMENTS

MANUFACTURERS

Pentax

Oberwerk

Fujinon

Oberwerk

SPECIFICATIONS

PCF III

LER

FMT-SX

Giant

and as sold notes

12 x 50

15 x 70

16 x 70

20 x 80






Field of View Mnfr.

4.2°

4.4°

4.0°

3.5°

Lens Coatings

Pentax SMC

Broadband FMC

Proprietary EBC

Broadband FMC



all glass/air surfaces

all glass/air surfaces


Prisms

BAK4

BAK4

BAK4

BAK4






Weight

2 # 2 oz.

3 # 2 oz.

4 # 12 oz.

7 # 12 oz.






Eye Relief

20

16mm

12.5mm

18mm

Exit Pupil

4.16mm

4.6mm

4.38mm

4.0mm






Focus

Center +/- R diopter

Center +/- R diopter

individual +/-L +/-R

Center +/- R diopter

Close Focus(by me)

(18 ft)

20m (65ft)

(65ft)

20m (70ft)






Mount Socket

Yes 1/4x20

Yes 1/4x20

Yes 1/4x20

mount shaft

Eye Cup

40mm soft rub fold

38mm soft rub fold

43mm soft rub fold

28mm hard rubber

Covering

Rubber Armored

Rubber Armored

Coarse Grained Vinyl

Painted Steel

Strap

camera wide

D rings wide

D rings wide


Lens caps

2obj + 2 eye tight

2obj + 1 eye tight

2obj + 1 eye tight

2obj + 2eye loose

Binoc Case

soft padded nylon

soft nylon

hard case w/strap

padded backpack

Accessories

None

cloth


tripod mount shaft






Price New

$149

$149

$599

$299

Price Used

$90 - $100

$99 - $129

$425 -499

$150 - $249

Shipping

$5

$8

$12

$20

The hard carry case that comes with the Fujinons is an excellent added feature, probably valued at about $30 if sold separately. It is fairly compact and offers excellent protection to the binoculars when they are stored in a footlocker for traveling. The grossly oversized backpack that comes with the Oberwerk 20x80s although padded is quite spartan. It has no secure latch, it s much too large to pack the binoculars safely without stuffing it with layers of bubble wrap and the straps are too flimsy to use it as a backpack.

The 20x80 is the first binocular I have seen with the adjustment screws readily accessible on the uncovered surface of the prism housings. Except for the Oberwerk 15x70, I bought all these binoculars used. Noted above are the 2nd hand price ranges that I have seen recently. Quality may vary considerably when buying 2nd hand and caution is urged.

QUALITY OF BUILD

QUALITY OF BUILD

Pentax

Oberwerk

Fujinon

Oberwerk

Minolta


PCF III

LER

FMT-SX

LER

Activa


12 x 50

15 x 70

16 x 70

20 x 80

7 x 35







Center pivot brace

tight

tight

easy

way too tight

tight

Pivot movement

excellent

VG

excellent

difficult

excellent

Interpupilary min/max

57mm / 72mm

58mm / 72mm

55mm / 77mm

60mm / 71mm

59mm / 73mm







Eyepiece bar rocking

no bar

little

none

little

none

Bar movement

excellent

VG

individual

VG

excellent







Focus mechanism

excel + lock

Good

very good

very tight

excellent

Focus stability

excellect

slightly loose

excellent

VG

excellent

Focus cold temp

excellect

very difficult

excellent

extreme difficult

excellent







Diopter mechanism

Excel + clickstop

fair barely suffice

excellent

poor insufficient

Ex clickstop

Diopter twist range

145*+ 145*-

60*+ 60*-

130*+ 120*-

60*+ 60*-

90*+ 90*-

Diopters + / -

7+ 7-

4+ 4-

L 7+ 6- R 7+ 6-

3+ 3-

4+ 4-

Diopter @ 20/30

1-

3-

L 0.5+ R 1+

4-

0.5+

Diopter E.R. mod.

2.25 +/-

0.9mm +/-

2.3mm +/-

1.3mm +/-

1.7mm +/-







Internal barrel

dull black

fine rib shiny cuts

fine rib matte black

shiny matte black

dull black

Internal baffel

yes

yes

yes

no

yes







Lens Coatings

SMC

Broadband FMC

EBC

Broadband FMC

FMC (darkest)

Coating color

purple

green/blue

green/blue/purple

green/blue

green

coating objective

light purple

lt. green / blue

lt. green purple

bright green


coating eye lens

lt. pur/green

blue

drk green/blue

blue


Lens reflections

little

most

almost none

most

almost none

reflections

7

6

10

5

9

color snow

w y

y

w

y








Prism Chords seen

1.5mm + 1mm

0

0

0

3.0mm + 2.4mm

Obstruction light loss

3% to 4%

none

none

none

7% to 8%







Collimation Screws

inaccessible

under armor

inaccessible

readily access

inaccessible

Pivot Hinges

The inter-pupilary pivot hinge motion was very good on all but the 20x80s. They were so tight that it requires a good strong pair of arms to move them. The 16x70s had the loosest movement, but still the movement was excellent and they did not move during use. The Pentax 12x50 pivot is smooth and stable, but the dropped shape of the prism housing comes very close together on the bottom and can be too narrow for your nose bridge if you have close-set eyes. I ve had a problem using some tripod adapters with the Pentax. A high profile adapter is needed to raise them up high enough or the binocs hit the mount seat plate on top of the tripod and won t come close enough together for the inter-pupilary distance of my eyes.

Eyepiece Bar

The eyepiece bar on the 15x70 s had some slop, allowing it to rock in & out. The Pentax 12x50s have a unique eyepiece mechanism. They are center focus, but they do not have an external eyepiece bar like the other center focus binoculars, eliminating any possible rocking motions. The Fujinons are individual focus eyepieces. All the binoculars held focus even under abnormal pressure.

Focus Mechanism

There are three different types of focus mechanisms represented in this group. The Pentax have center focus with internal movements. The center focus dial has a lock that is a nice feature. The right diopter has a click stop adjust that provides a very positive tactile feel and in my opinion improves the ability to find and hold the right adjustment. The Pentax have one of the best focus mechanisms I ve ever used. The Fujinons have individual focus eyepieces. Compared to any other tripod-mounted binocular, I find these Fujinons just as easy to use as a center focus with a right diopter adjustment. The Oberwerks have center focus with a right diopter adjustment. Except for the 20x80s, which are very difficult to turn, all the binos had a focus mechanism that operated smoothly throughout the entire dial range. Minolta Activa 7x35s and Nikon Kestrel 10x50s have a right diopter with click-stop adjust similar to the Pentax.

Especially when cold, it was necessary to grip the 20x80 center focus dial top and bottom with finger and thumb to turn the dial. Images are difficult to get really focused sharply. You need to really work at focusing. The Pentax12x50 and the Fujinon 16x70 came to a fine point focus very easily.

Focus Diopter Range

The diopter adjustment on the Pentax and the Fujinons is generously substantial, allowing turning the right eyepiece, or in the case of the Fujinon both eyepieces, thru 1/3 full turn or more each way. The diopter adjustment on both the Oberwerk 15x70s and the 20x80s allowed turning the right eyepiece only thru 60°+ or 60°- (1/6 turn each way). I have indicated in the table above what the reading on the diopter adjust was when I focused to my right eye which is corrected to 20/30 with glasses. For me, even with my glasses on, the 15x70s required a setting well into the minus diopter range to focus. Without glasses, I was not able to use either of the Oberwerks. With the 20x80s, what is even more significant is that even with my glasses on and my right eye corrected to 20/30, I have the right diopter adjustment set at the full 60° minus setting and I still cannot achieve perfect focus for that eye.

Internal Barrel Reflections Baffles

Baffles help block stray light that is scattered towards the internal sides of the barrels. It is desirable to control light scatter on the inside of the barrel by using non-reflective coating, baffle rings and baffle ridges. Reflected light inside the barrel will reduce the overall contrast of the objects viewed.

If the image of a bright object is moved towards the edge of the binocular fov, the Oberwerk 15x70s and the Oberwerk 20x80s show bright reflections down the inside of the barrel. The Oberwerk 15x70s have baffle ridges cut into the inside of the barrel, however every cut leaves a bit of shiny metal showing and this may be acting like a reflective surface lining the insides of the barrel. The Oberwerk 20x80s barrel interior is covered with a black coating that seems to be completely impregnated with a silvery white reflective powder. The insides of the barrels reflect light almost like a mirror. By far the Oberwerks had the most internal reflections of any binoculars in this group tested. Minolta Standard XL12x50s and Orion Ultraview 10x50s showed minor reflection off the inside of the barrel. No other binocular that I ve tested shows any light reflection off the inside of the barrel.

Fujinon 16x70 barrels are matte finished half way down the barrel. At that point there is a baffle ring. Beyond that the barrel has fine matte black ridges. No other binocular had this high degree of baffling. Orion Vista 8x42s had a similar barrel, half matte, half ridged, but there is no baffle ring. Pentax 12x50s have a matte finish in the barrel with two baffle rings. Minolta Activa 7x35s have a matte black barrel with a baffle ring.

Coatings

Uncoated lenses may pass only 93% to 96% of the entering light through each lens element. A binocular with four uncoated surfaces is passing only about 75% to 85% of the available light. The rest is reflected back off the glass surfaces. The remaining light that does not pass thru the lens can scatter off the insides of the barrel and cause glare in the image. A single coating of Magnesium Fluoride on each optical surface can increase light transmission up to 97% or 98+%. Single coated Mg F lenses can look blue. The best modern coatings and multi-coated lenses can increase light transmission up to 99+% for each surface, almost negating light loss due to reflections. Multi-coated lenses can look blue, green or purple, depending on the coating used.

The view through binoculars with better coatings provides the eyes with a noticeable increase in the ease of seeing. One of the finest coated binoculars I own, by outward appearances and by image fidelity, is the Fujinon FMT-SX 16x70. The Fujinon coatings have a green/purple/blue color and show nearly no reflections. Orion Ultraview 10x50s have excellent green/purple/blue coatings and show nearly no reflections. Likewise, Orion Vista 8x42 have green/purple/blue coatings and show almost no reflections. The Pentax 12x50s have purple coatings that show little reflection.

Both Oberwerk 15x70 and 20x80 binoculars have green/blue coatings and at least one very bright reflection shows when looking into the objective end. In fact, the reflections off the Oberwerk lenses shows bright detail and color of the object reflected, more so in the 20x80s. By comparison, looking for a reflection in the Fujinon, all you see is a muted dark outline of the object reflection, no detail and no color what-so-ever.

Nikon Sky and Earth Kestrel 10x50s have green/blue coatings similar to the Oberwerk, but they show only moderate reflections. Minolta Activa 7x35 coatings are dark green and show almost no reflections. As compared to a pair of Minolta Standard XL 7x35 with light blue coatings, the Activa view has higher contrast, making it easier to see faint objects.

Reflections off the objectives at any particular angle may not be an accurate indication of the ability of the coatings to perform on-axis thru the lens system. Probably the best test is to view the brightest objects and look for reflections or haze thru the eyepiece. Reflections or glare through the eyepiece is a fairly good indication of poorly coated or uncoated optics. The 20x80s showed ghost reflections of Jupiter and Venus.

The combination of proper baffling and improved coatings provides increased light to the eye, less scatter, higher contrast and an overall more pleasing view. Comparing binoculars side by side, it becomes readily apparent the improvement afforded by those with better coatings. Faintest objects, not seen in binoculars with lesser quality coatings, more readily stand out from a darker sky background.

Prism Light Cutoff

Prism light cutoff can be seen by looking into the eyepiece end to check for dark chords at the very edges of the exit pupil image circle. As opposed to the diamond effect described as light fall off towards the edges of BAK-7 prisms, this is complete cut off of light due to impairment in the light path.

The chords are the images of prism deficiencies infringing on the circular light path. They are showing blocked light that entered the objectives but does not reach the eye lens. There are two deficiencies that may cause this problem. Prisms may be either too small, not completely spanning across the prism shelf hole, in which case the edge of the prism misses a portion of the entering circle of light, or prism sides may protrude into the light path in the barrel, blocking a portion of the entering light before it reaches the prism. Most times you can look down thru the objective and see whether the sides of the prisms infringe on the light path.

In this group, only the Pentax 12x50s show impairment of the light path due to undersized prisms. The light loss is calculated at 3% to 4%. Half of all the binoculars that I ve tested show prism light cutoff. Some of these are the Minolta Activa 7x35 (7% loss), Orion Ultraview (6% loss) and Orion Giant 16x80 (5%loss).

Collimation

Collimation is the alignment of the two images along the optical axis such that two images merge to make one. Nothing will turn you off from using a pair of binoculars as quickly as a poorly collimated pair of objectives. If the severity of the problem is great enough, you will very quickly get an uncomfortable or even dizzy feeling. As a quick test you can try, while looking through the binoculars, if you cover one lens with your hand and then uncover that lens, you can see if the images are merged or momentarily separated. If the separation of the images is severe enough, this quick test will show it. If they are only off by a small amount, you may not be able to see it using this simple test.

Once it is determined that collimation error is present, you need to determine if it is acceptable or needs adjustment. An excellent way to check the degree of collimation error is to sight on a wide double star of known separation. For this purpose I use Nu v Draco 62”, in the head of Draco. It is easy to estimate error when you are looking at an object in the same field that gives you a measurement baseline and this 1 arcmin double star is easy to see in any binocular.

Generally there are three collimation errors to be concerned about. For all misalignments the allowable error is smaller as magnification increases. My actual experience is that my eyes will not allow separation as large as defined by these limits, especially at magnifications of 15x and higher.

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 and only 3 arcmin at 12x to 15x.

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 and 8 arcmin at 12x to 15x. Most eyes can readily accommodate this error, but these limits I find a little too wide.

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.

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.

Both my Fujinon 16x70 and Pentax 12x50 have slight collimation errors, however they are both less than the allowable limits and my eyes easily merge the images. The Orion 16x80s were collimated in exact alignment. The Oberwerk 15x70s have collimation adjustment screws accessible just under the rubber armor. The Oberwerk 20x80s have the adjustment screws readily accessible on the outer surface of the steel prism housing. The 15x70s seem to lose collimation rather easily. I have adjusted the 15x70s several times and before too long, I observe that they need adjustment again.

I have written an in-depth article on binocular collimation and the implications of improper adjustments. I encourage you to read that article before attempting binocular collimation. “Collimating Binoculars” http://www.cloudynights.com/howtos2/binoc-collimatin.htm

Overall Quality

The Pentax 12x50 are outstanding binoculars in their price range. The mechanical operation and the optics make them a real pleasure to use. A wide range of adjustment, fine mechanical motions and clear sharp instant images add up to a top quality performer. The Fujinon 16x70 are a higher end binocular and the feel, mechanical operation and superior images reflect quality. A look through the Fujinons side by side with any other binocular I own shows improved contrast and better resolution, probably a direct result of better coatings and baffling. Although the Oberwerk 15x70 perform fairly well, they do not see as well as the Fujinon and there is room for improvement of the mechanical operation, adjustability, coatings and baffling. Still they are a great bargain. The Oberwerk 20x80s, while not near the equal of the Fujinons in quality, do have impressive image scale; but even more so than the 15x70s.

HAND HELD IMPRESSIONS - ERGONOMICS

ERGONOMICS

Pentax

Oberwerk

Fujinon

Oberwerk

Minolta

Nikon


PCF III

LER

FMT-SX

LER

Activa

S&E Kestrel


12 x 50

15 x 70

16 x 70

20 x 80

7 x 35

10 x 50








Length = in. (mm)

7.0 (178)

10.3 (260)

10.9 (277)

17.0 (432)

4.8 (121)

7.1 (180)

Width = in. (mm)

7.4 (188)

8.0 (203)

8.5 (216)

10.0 (254)

6.3 (159)

6.4 (163)

Height = in. (mm)

3.5 (89)

3.3 (83)

3.9 (99)

4.6 (117)

3.0 (76)

3.5 (88)








Weight lbs. (kg)

2.1 (0.966)

3.1 (1.420)

4.75 (2.160)

7.75 (3.520)

1.75 (0.795)

2.0 (0.909)








Hand Fit

excellent

good hold

fair hand hold

no hand fit

excellent

good hold








Focus Dial

reach to

far reach to

can't reach

can't reach

finger on

reach to








Hand hold

some shake

lots of shake

lots of shake

not holdable

very little shake

some shake

Hand stability

easily braced

braceable

mount only

mount only

hold one hand

hand hold

Size, Weight

The 20x80s are huge and very heavy. These binoculars are not intended to be hand held. The 15x70s seem much smaller in actual comparison than the specifications would indicate. Oberwerk 15x70s are long but light and are much more like using big 50s than like using small 80s. Although it was difficult to hold the 15x70s still, I was able to daytime view for short periods and it was easy enough to brace these while hand holding. The Fujinon 16x70s are heavy binoculars. I bought these with no intentions of using them hand held. I have numerous stable tripods and several parallelogram mounts, one of which is employed with the Fujinons.

Various 35s and the 50s are easily hand held and I have often taken either of this size on day hikes or backpack trips. 35s can be used for hand held astronomy. 50s can be hand held for astronomy, but in order to get the maximum benefit it is best to mount them, especially once you get up into the 12x range. Even though the Pentax 12x50s are surprisingly lite, I could not hold them steady enough to see any detail. I could see much more detail with the 12x50s when they were braced and even more detail when they were mounted. Except as noted, all of my night sky viewing was tripod mounted.

Focus Reach

The focus knob on the Pentax 12x50s and the Oberwerk 15x70s could be easily reached without moving the hands from the holding position. On the 15x70s, the forefinger must reach out from the holding position and focus was achievable while holding although stillness of the image was a problem. On the Pentax 12x50s, the index finger falls right on the focus knob. The Pentax 12x50s were easy to use hand held, but bracing the elbows helped settle the image. On the Oberwerk 20x80s, the focus dial cannot be reached while holding the binoculars and focus cannot be adjusted successfully without the binoculars being still. Therefore, the 80 s must be mounted to adjust focus. The Fujinon 16x70s, with individual eyepiece focus, places the focus out of reach even for those with stout arms that might attempt to hand hold these moderately heavy binoculars. Orion and Nikon 10x50s are easily focused while handheld. The Minolta Activa 7x35s are so small and fit the hand so well they can be held with one hand and focused with one finger.

EYE RELIEF

Eye relief, as the term is most commonly referred to, is exit pupil image distance, the distance at which the exit pupil is sharply focused and fully illuminated. Eye relief wanted by the user is how much distance is desirable to comfortably place your eyes behind the binocular and see the fully illuminated exit pupil and 100% of the fov.

Lens Recess

All lenses are recessed, some only a little, but others are recessed deeply. If the lens is recessed deep, the depth to the lens is using up a large portion of the eye relief and less eye relief is available to the user. An example is the Orion Giant 16x80. Exit pupil image distance = 17mm. Lens recess = 7mm. Available usable eye relief therefore is only 10mm.

Eye Cup Depth

Lens recess was measured both with the cups folded down and with the cups up. When the cups are up, usually the eye is placed at about the proper distance for correct eye relief for those not wearing glasses. This helps eliminate the blackout problem that some people experience. The cups are folded down for those wearing glasses and good eye relief is necessary to see the full field of view

EYE RELIEF

Pentax

Oberwerk

Fujinon

Oberwerk

Minolta

Orion

Nikon


PCF III

LER

FMT SX

Giant

Activa

Ultraview

S&E Kestrel


12 x 50

15 x 70

16 x 70

20 x 80

7 x 35

10 x 50

10 x 50









Eye Relief Published

20mm

16mm

12.5mm

18mm

22mm

22mm

20mm

Field Measured Data








Exit Pupil Image Focus

20mm

17 mm

18mm

18mm

20mm

18 mm

21mm

Eye Cup Up Recess

16mm

13mm

15mm

11mm

15mm

14mm

16mm

Eye Cup Down Recess

3mm

2mm

9mm

1 mm

4mm

4mm

3mm

Usable Eye Relief

17mm

15 mm

9mm

17mm

16mm

14mm

18mm

Diopter Eye Relief mod.

2.25 +/-

0.9mm +/-

2.3mm +/-

1.3mm +/-

1.7mm +/-

1.0mm +/-

2.3mm +/-









Eye Cup Eye Opening

40mm

38mm

43mm

34mm

40mm

31mm

40mm

Eye Lens Diameter

18mm

22mm

22.5mm

24.5mm

22mm

22mm

18.5mm

Eye Cup Lens Opening

25mm

21mm

24mm

28mm

30mm

31mm

24mm

Exit Pupil fld measure

4.4mm

4.67mm

4.4mm

4.0mm

5.0mm

4.8mm

5.0mm

Measured Eye Relief

Eye relief (exit pupil image distance) was measured by attempting to bring the exit pupil image to a clear focus on a white card, then measuring the distance from the lens to the card. I found this to be a difficult task and consider the accuracy of hand held measurements of Eye Relief to be +/- 2 mm. Relative to each other, the measurements are fairly accurate. Once Exit Pupil Image distance is known, Usable Eye Relief equals the distance back to the eye guard with eyecups folded down, not back to the lens. This is a significant number for eyeglass wearers and in some cases is substantially less than the advertised eye relief.

Viewing With Glasses

In all cases the eyecups must be folded down to view with glasses. The Pentax 12x50s easily allowed 100% view to the edge of the field stop. The Oberwerk 15x70s allowed 100% of the FOV. The Oberwerk 20x80s provided a 100% view. The field stop could not be seen in the Fujinon 16x70s, but by looking around in the FOV, I could see it all around. The Fujinons provided the least, about 90% of the FOV while wearing glasses, probably a result of the recessed lenses using up so much of the available eye relief. I had similar results of about 90% fov visible with Orion Giant 16x80s. They also had a deeply recessed eye lens that used up a large portion of the eye relief.

Viewing Without Glasses

All binoculars could be used with cups folded down to view the edge of field, as long as you found a distance to place the eyes that didn t experience blackouts. With the cups up to shield the eyes and place the eyes at correct eye relief distance, the Pentax 12x50s allowed seeing only 90% of the FOV. The Oberwerk 15x70s showed about 90% of the FOV and the 20x80s showed 90%-95% of the FOV. The Fujinon 16x70s have short eyecups when folded out that place the eyes at a distance that allowed seeing 100% of the FOV. Orion s 10x50 Ultraview reduce FOV to 80%-90% when the cups are turned out. The Minolta Activa 7x35s allowed seeing the entire field.

Eye Cups Function

The 15x70s have very comfortable soft rubber fold -down eyecups that pop up or down easily. They can be pressed against the eye socket and have some give. The Pentax have similar comfortable soft cups, but they are very difficult to fold up or down. The 20x80s were supplied with a hard molded eyecup with wings that could not be used with glasses. These needed to be removed to use glasses and when removed they leave the plastic eyepiece housing exposed permitting glasses to rub up against an undesirable surface. I needed to replace it with a soft round eyecup that could be used with glasses. The eyecups on the Fujinon 16x70 are wide enough to cover the eye socket, but they are shallow and not too soft, similar to the Orion Giant 16x80 eyecup. All of the eyecups if used, place the eyes just within the range of optimum exit pupil illumination.

I found in practice that the 15x70s, 16x70s and the 20x80s were used with glasses on and eyecups folded down. I was not pressing up against the eyecups to view. In most instances I was several mm back away from the eyecups and viewing was fine. However this did prevent me from seeing the field stop.

I liked the Pentax 12x50s used without glasses. They feel comfortable up against the eyes and I can see the entire field. In the bigger binoculars, with powers of 15x and up, my astigmatism is noticeable and I found it better to view with my glasses on.

Exit Pupil

I measured exit pupil by the size of the image on the eyepiece lens. I used calipers and tried to sight the image. The best I can say is that all measured +/- a few tenths of mm. I m not using this measurement for any calculations.

I should mention here that I have used all binoculars with exit pupils of 5.25mm or smaller. Maximum brightness would be achieved with an exit pupil equal to approximately 7mm, the average diameter of the dark-adapted eye. Extended objects would be seen easier with maximum brightness. The trade-off is lower magnification. Sometimes there is a compromise point where a little higher magnification and a little less brightness of the image may result in making it easier to see not only point source objects but also some extended objects. This may not always be the case.

FOCAL LENGTH

Binoculars are Fast Achromats

Although this is not a particularly useful measurement in a fixed system, knowing how to calculate the focal length of binoculars may help you to understand a little more about how fast the optics are operating. The short f-numbers of these systems certainly help explain why chromatic aberration can be present. It helps to see a diagram. Several of the books referenced in the credits at the end of this article show a cut-a-way diagram of a porro prism binocular. I have not included a diagram here.

In binoculars, the same as in telescopes, magnification is focal length of objective (Fo) divided by focal length of eyepiece (Fe), or mag = Fo / Fe. Also, Fo + Fe is the total length of the light path. So, we know two formulas with the same terms and we know magnification. We need to find the length of the light path, Fo + Fe. Then we can solve the problem.

Measure the outside lens-to-lens distance of the binoculars. Closely estimate the length of the light path through the prisms and add this to the measured lens-to-lens distance. I have had several binoculars opened up and had the opportunity to measure the light path through the prisms. The prism light path in my smaller binoculars, those with mag. of 7x to 12x, measured 90mm to 100mm. The light path in only the largest of the giant binoculars, 16x70, 16x80 and 20x80, with very large prisms measured 120mm to130mm. These two measurements added together, lens-to-lens + prism path, is the total light path, the total of Fo + Fe. For a Nikon 10x50 the Fo + Fe = 254mm.

Fun With Math

Now we have two formulas. We will use the Nikon for our example. We know Fo + Fe = 254 and we have Fo / Fe = 10. Since magnification is given as 10x, it s easy to understand this second formula will result in a Fo = 10x longer than Fe. If we solve this 2nd formula Fo / Fe = 10, then Fo = 10 Fe. Now substitute this answer for Fo into the first formula, so Fo + Fe = 254 can be written as 10 Fe + Fe = 254. Therefore 11 Fe = 254. So Fe = 254 / 11 = 23.1. Now then 254 Fe = Fo, or Fo = 231. A check of the results is Fo / Fe = 10. Our check, 231 / 23.1 = 10 shows the answers are correct.

All of this just to get the f number of the binoculars? Yes, since now that we know the Fo, we can divide Focal length of objective by the Diameter of the objective and we get f, the speed of the binocular system. In this example it is Fo / Do = f, or 231 / 50 = 4.6. These Nikon 10x50 binoculars are operating at f4.6. Seven out of twelve binoculars I tested are operating within the narrow range of f4.6 to f4.8. Orion Ultraview 10x50 and Orion Vista 8x42 were both at f4.3. Oberwerk 20x80s were at f5.6. Other checks (exit pupil) will prove out these answers. Exit Pupil = Fe / f, so 23.1 / 4.6 = 5.02. Compared to E.P. = Do / mag, 50 / 10 = 5, it checks.

Two Things You Need to Know

Having gone through this whole exercise, you need to know only two things to help you determine the f of binoculars. First, the total length of the light path, which you must accurately measure, and second, that the shortest form of the formula above, for any binocular, will always be F eyepiece = Length of Light Path / (magnification + 1). The rest is simple subtraction and division.

MAGNIFICATION

The next two sections of this article, Magnification & Contrast and Resolution, developed into a separate article. Through further research, the information in that separate article is substantially expanded beyond what is presented here. I recommend the reader also refer to the article “Binocular Performance”for the additional information provided there. edz

Importance of Magnification

I wanted to determine how much magnification was contributing to the overall gain in seeing. Based on my experiences at the telescope, I expected power to have a significant benefit on faint objects. When at the telescope and you want to see deeper into a cluster you can t always go looking for a larger aperture scope, you simply put in a higher power eyepiece. With a given telescope aperture, I have noted many times at higher powers that I can see more and fainter stars in clusters. I have noted the same experience with binoculars. To test this I tried various binocular magnifications all at the same aperture.

The Mask Test

The mask test is simple, although tedious. Use various sizes of binoculars on numerous star fields and deep sky objects at full aperture and again at masked aperture. A cardboard mask 50mm in diameter was placed over the objectives of the larger binoculars. I have included views thru various 70s and 80s at full aperture and then masked at 50mm. I ve also used 3 different 50mm binoculars in the comparison. The primary requirement is keep to the same star field boundaries for every binocular and record exactly what is seen. Nearly all of the comparisons were done side by side under the same conditions by the same observer.

For this 2nd binocular mask test I started with 10x50s and 12x50s. Cardboard masks 50mm in diameter were placed over the objectives of the 15x70s, 16x70s and the 20x80s so a comparison could be made to the 50s. Two of the binoculars that were used in the 1st test, the Orion Ultraview 10x50 and the Oberwerk LER 15x70, are again used in the 2nd round of tests. Previously, I did this test with 10x50s, 15x70s and 16x80s at full aperture and then again with the 70s and 80s masked down to 50mm.

Star Fields

Orion

Pentax

Oberwerk

Oberwerk

Fujinon

Fujinon

Oberwerk

Oberwerk


Ultra

PCF

LER

LER

FMT-SX

FMT-SX

Giant

Giant


10 x 50

12 x 50

15 x 50

15 x 70

16 x 50

16 x 70

20 x 50

20 x 80

50mm Mask



masked

masked

masked




I 4665

20

27

26

28

28

29

Lyra area3

16


23

25

26

28

Pleiades 3 SE

24

26

24

27

30

31

27

28

Pleiades bowl

17

18

20

22

21

Perseus Assoc



33

35

35

M44



63

71

71

80

64

73

NGC2264



13

17

18

19

16

17

Orion1

25

34

34

35

37

40

36

39

Orion2

8

14

15

18

18

20

19

21

Orion3

11

13

14

17

17

The aperture mask results show the # of stars in a given field increases considerably with a magnification jump from 10x to 12x to 15x, 16x or 20x. In one case, at masked aperture, there was an increase from 25 to 40 stars within the same field. At 15x the number of stars in the given fields increased by about 40% over 10x50. At 16x and 20x the number of stars increased by about 50-60% over the 10x views.

With the mask, the sky background at 15x, 16x and 20x was slightly darker. The binoculars, which at full aperture operate at f4.7 to f4.8, masked were operating at f6.8 except the 20x80s. They are normally about f5.6 and masked they were operating at f9. (Please refer to the article “Binocular Performance”for an expanded discussion of the affects of the aperture mask).

The 10x50s were easily surpassed by every higher magnification. Stars that were at the limit of vision in the 12x50s or 15x70s would readily pop into view at higher magnifications. There were some results that showed more stars at 16x than at 20x. However, the mask test began to show the relative weight of magnification vs. aperture.

Magnification vs Aperture

In open clusters, the difference in the number of stars seen at full aperture versus masked to 50mm was only a few stars, generally on the order of only 1 to 3 stars more. About 80%+ of all the additional stars beyond what the 10x50s could see were viewed with the apertures masked at 50mm, indicating aperture is not responsible for the added seeing but magnification is likely responsible. The conclusion reached here is that most of the additional stars seen in a field are a result of the increase in magnification and only a marginal increase is due to aperture.

It should soon become apparent, the primary benefit aperture may be contributing is maximum brightness for the viewing of all objects but especially for extended objects, and maximum brightness is achieved at the lowest magnification. For all other purposes, the maximum potential ability of aperture is not fully utilized and any increase in magnification provides significantly greater ability to utilize that potential, giving greater steps up in performance.

Magnification vs Limiting Magnitude and Maximum Resolution

In telescopes, one of the most important considerations is aperture. Along with the maximum amount of light that can be gathered, aperture controls the limiting magnitude and maximum resolution. It is the ability of raising the telescope to optimum magnification that allows those two performance limits to be achieved. If the telescope is used at lower magnification, maximum light can be gathered, but limiting magnitude or maximum resolution may not be achieved. Magnification really helps us to see.

In binoculars, the magnification is generally so low that the limiting magnitude and maximum resolution abilities of the objective lens cannot be reached. Lenses of 70mm to 80mm have a maximum resolution of between 1.5 to 2.0 arc seconds. I have recorded views on double stars with various 50mm, 70mm and 80mm binoculars and results I get indicate maximum resolution by the best binoculars is in the 7” to 10”range.

This helps explain why we get so much significant improvement in binoculars by increasing magnification. Along with increased image scale, higher magnification utilizes more of the ability of the available aperture. As an example, on an exceptionally still clear night, the AB component of the Trapezium at 8.7”was resolved in the 20x80s at full aperture and was seen also masked at 20x50, but was not seen in any other binocular of lesser magnification. It was the magnification of 20x, not the aperture of 80mm that provided just a bit more ability to see allowing easier splitting of this close double than could be seen with the 16x70s or the 15x70s.

Baseline Index

There are a number of references out there to ‘Binocular Indexes , some of which simply multiply magnification x aperture to arrive at an Index Value. Based on the test results noted above, I tend to agree with Alan Adler that magnification holds much greater importance than aperture and that indexes should account for this importance. Alan uses an index based on magnification x square root of aperture. This seems definitely headed in the right direction.

Binocular Index

7 x 35

10 x 50

12 x 50

15 x 70

16 x 70

16 x 80

20 x 80

Magnification

7

10

12

15

16

16

20

Objective Diameter

35

50

50

70

70

80

80

Sq Root Obj Dia

5.9

7.1

7.1

8.4

8.4

8.9

8.9

Adler Index

41

71

85

125

134

143

179

increment increase

1.0

1.7

1.2

1.5

1.1

0.7

1.3

The Adler Index is important in that it provides a magnification/aperture baseline across all makes and models that show the size performance order of binoculars. We need this to begin a comparison that includes other attributes. It shouldn t be difficult to agree that not all models in a given size are built with the same quality attributes. Therefore, due to variations in quality, they may not hold the same rank in a performance index.

CONTRAST and RESOLUTION

There are many additional binocular performance attributes that can be considered when trying to rank performance. However, I think the two things that contribute most to improved performance are contrast and resolution. These two attributes are affected by the quality of the glass in the optical system, the coatings on all glass surfaces and baffles built into the binocular. The following method is directed towards accounting for coatings and baffles that impart greater benefit to better binoculars.

Impact of Coatings and Baffles

The better performance of the Fujinon 16x70s, due to the improved qualities of coatings and baffles, seem to skew the magnification mask data. Whether the Fujinons were masked to 50mm or not, as compared to 16x80s and 20x80s, they were picking up more stars in wide-open clusters and in tight clusters they were resolving more stars. Likewise, I think the 10x50 Ultraview vs. the Nikon 10x50s and the 12x50 Pentax vs. the Oberwerk 15x70s both showed improved performance due to better baffling and coatings. There must be some benefit of higher quality lenses, coatings and baffles. The introduction of these examples into the mix shows the need to modify the base index, so it includes a factor for the impact of the quality of coatings and baffles.

Image scale can be deceiving. Higher magnification, at first look, can give you the impression that you are seeing more. You are seeing it bigger, and indeed you may be seeing more, but not always. In some cases, higher magnification provides greater ability to see, for instance when splitting close doubles. But better contrast and resolution, provided by higher quality coatings and baffles, at a slightly lower magnification may result in seeing more.

Resolution of Fine Detail

In addition to maximum resolution, the ability to split close objects that we discussed earlier, we need to discuss resolution of fine detail. We have already established due to low magnification we cannot utilize the maximum ability of the aperture. We must rely on superior contrast to provide us with the optimum utilization of the system being used for viewing.

Globular clusters, as seen in telescopes, are a perfect example of that class of objects that require contrast to begin to resolve. They appear to glow because the size is so small and the large number of stars is so closely packed they sometimes cannot be resolved fully. In binoculars there are a great many small open clusters that take on a like appearance. In some ways these objects are similar to fine planetary detail. Contrast is required to differentiate the fine detail from the nearly equal light in the immediate vicinity. Although the object may be bright, the light gradient across the object may be low. This makes it more difficult to pick out or resolve any detail. The ability of the binocular system to resolve is already limited by overlap of the broader diffraction fringes because it operates at magnification well below potential resolution. So contrast is needed to resolve the small differences in light gradients.

Need for Contrast

A good example is M36. Read what Stephen O Meara wrote about M36. “Its 15 brightest members (between 9th and 11th magnitude) shimmer against a tight, hazy background caused by the feeble light of dozens of 13th and 14th magnitude stars.” And about M37 he writes, “At 23x this cluster looks like a finely resolved globular cluster a marvelous illusion. Indeed, Luginbuhl and Skiff likened it to a “broken down view” of M22…” “About 500 of these suns are brighter than 15th magnitude, and they are spread across 15 …”

Sky Catalogue 2000 lists M36 as 60* within 12 diameter, the brightest star being 8.8mag. Luginbuhl and Skiff states it is more concentrated towards the center. Burnham s Celestial Handbook states that M36 has 15* ranging from 8.8mag to 10.6mag. On one occasion, I noted seeing 6* in the 15x70s, 8* in the 20x80s, but I counted 13* in the dense core with the Fujinon 16x70s. The results here indicate stars resolved in dense open clusters increased as contrast and resolution improved.

Resolving

Orion

Pentax

Oberwerk

Oberwerk

Fujinon

Fujinon

Oberwerk

Oberwerk


Ultra

PCF

LER

LER

FMT-SX

FMT-SX

Giant

Giant


10 x 50

12 x 50

15 x 50

15 x 70

16 x 50

16 x 70

20 x 50

20 x 80

50mm Mask



masked


masked


masked











M35


13

13

14

21

30

18

20

M36




4

7

8


4

M36


6


6


13


8

M37


2


3


8



Saturn's Rings

no

no


yes


yes


yes

Saturn's Rings


no


partly


resolve


resolve

Jupiter/moons


2b/4m 1dif


2b/4m 1easy


2b/4m 1<30"


2b/4m 1<30"

Split Mizar


suspect


no


yes


yes

Trapezium




3 difficult


4suspect


4suspect

Trapezium


3suspect


3 barely 4no


3easy 4elong


4resolve

Trapezium







4resolve

4resolve

E 953 Mon




no


elong


elong/res

Contrast Allows You to See More

Magnification improves the ability to resolve because objects are seen at a larger image scale. However, the qualities of contrast and resolution in an optical system are best observed when you have side by side comparisons and the smaller optics are seeing equal to or more than the larger optics. This most definitely cannot be due to magnification and I believe it is attributed to contrast, the result of better coatings and baffling.

In a perfect system, a larger diameter objective imparts a smaller diffraction fringe around every point of light in an image and the result is we can see finer detail. What qualities of the lens then allow the smaller objectives to see more? It is not resolution but contrast, the quality that allows seeing more in the detail provided by the rest of the system.

Contrast and resolution are enhanced with the introduction of improved multi-coatings and effective baffling. Binoculars that do not incorporate the best of these features do not measure up to the performance of similar sized or sometimes even smaller binoculars that do. That s why models such as The Pentax 12x50s or the Fujinon 16x70s, with better coatings and baffling, perform at a much higher relative position on the index than do higher magnification binoculars with a larger aperture.

Performance Index

Coatings and baffling have been proven to impart significant impact to the performance ability of binoculars and must be accounted for in any comparative performance indices. In the following index, I account for the various qualities or absence of quality in coatings and baffles by modifying the base index value with these adjustments:

+ 10% for best coatings 0% coated - 10% some lack of coatings

+ 10% for best baffles 0% baffled - 10% some lack of baffles

Index

Minolta

Orion

Pentax

Oberwerk

Fujinon

Oberwerk


Activa

Ultraview

PCF III

LER

FMT SX

LER

Binocular

7 x 35

10 x 50

12 x 50

15 x 70

16 x 70

20 x 80

Magnification

7

10

12

15

16

20

Objective Dia mm

35

50

50

70

70

80

Sq Root Obj Dia

5.9

7.1

7.1

8.4

8.4

8.9

Adler Index

41

71

85

125

134

179








Coatings

10%

10%

10%

-10%

10%

-10%

Baffles

0%

0%

0%

-10%

10%

-10%








Performance

46

78

93

102

162

145






















Index

Minolta

Orion

Nikon

Minolta

Orion

Bear


Standard XL

Vista

Kestrel

Standard XL

Giant

Custom

Binocular

7 x 35

8 x 42

10 x 50

12 x 50

16 x 80

25 x 70

Magnification

7

8

10

12

16

25

Objective Dia mm

35

42

50

50

80

70

Sq Root Obj Dia

5.9

6.5

7.1

7.1

8.9

8.4

Adler Index

41

52

71

85

143

209








Coatings

0%

10%

0%

0%

0%

0%

Baffles

0%

10%

0%

0%

10%

0%








Performance

41

63

71

85

157

209

I have assigned a value to Coatings and Baffles based on the notes in the Quality of Build table. I was tempted to start using % values in single increments all the way from 10% to +10%. I don t think that is necessary, especially for a relative index. Instead I simply moved the rank up or down in this way. If there are any reflections off the objective 10%, if the coatings seem average then 0% change, if there is little or no reflections at all +10%. Likewise if the barrel has anything bright or shiny inside or has no baffle 10%, if the barrel is baffled or has a dull finish 0% change, if the barrel is effectively ribbed, or has an effective matte finish with a baffle +10%.

I am surprised at how closely I think these values represent the performance rank of all these binoculars based on actual use, except maybe for the 25x70s. Even though they had bigger image scale, I don t recall the increased magnification being enough to outweigh the performance of the Fujinons. The 25x70s may be showing that this index puts slightly to much emphasis on magnification and not enough on quality. My opinion is that the Fujinons were the best performers. Just as any other index would, I believe this index is only a good indication of relative performance.

For any binoculars you want to compare, you can apply this index and get a pretty good indication of their relative performance. Information on coatings and baffles is usually available. Off the shelf, with only a quick outward inspection and without much or any testing, the expected values for these features can be assessed. Certainly, the opportunity to look down the barrel at the glass, the coatings and insides has a tremendous benefit in determining ranking. Generally, the quality level of binoculars is fairly well known and it seems the values for coatings and baffles follows pretty closely in line. Simple binocular indexes that do not account for these features are leaving out qualities that have a significant influence on performance. Applying this info to an index gave me a much better indication of relative performance of all my binoculars.

OBJECT FINDING FIELD OF VIEW

True Field of View = Tfov

TFOV is how much an area of sky you can see at once. FOV is measured in degrees. From horizon to horizon is 180 degrees. Human eyes can take in about 50° to 60° degrees of sky without any optical aid. The distance across Orion's Belt stars is a little less than 3 degrees, considered a small field of view for binoculars. When panning the sky looking for objects it is much better to have a 4° to 6° Tfov at a minimum. Oberwerk 15x70 have a 4.4° Tfov. Orion Ultraview 10x50 have a 6° Tfov. Minolta Activa 7x35 have a 9°+ Tfov.

Even a 3.5° Tfov makes it difficult to orient to the night sky. I have used two different binoculars with fields of view of 3.3° and 3.2°. They were more difficult to use than even 4.1° (Pentax 12x50) and 4.4° (Oberwerk 15x70). For a while I used a pair of 25x70 binoculars that had a 2.6° fov. The 25x70s were mounted for all viewing and although magnification was outstanding, the fov was restrictive and made it very difficult for finding objects. Binoculars with a field of view smaller than 2 degrees would be similar to looking thru a telescope eyepiece. You would need a finder scope mounted on top of the binoculars to find anything.

Maximum Field of View = TFOV

The True FOV is related to the ease of finding objects. A certain amount of perspective is necessary to find objects and a larger FOV provides more perspective. However, FOV and magnification go hand in hand for finding and a low power binocular with a wide FOV can be just as much a disadvantage as high power with a small FOV.

True measure of the FOV was obtained by noting the limits of star fields and referring to SkyAtlas 2000.0 for a measure of the field limits. Several star fields were used to test for maximum viewable field and measures were taken more than once to confirm results. The data recorded as ‘Field Measured Data separation in mm is the measurement in mm determined from SkyAtlas 2000.0 of the actual limits of view recorded from the eyepiece. When the results of observations varied, the largest readings were used as long as that reading was confirmed by at least one other observation.

Maximum TFOV

Orion

Pentax

Oberwerk

Fujinon

Oberwerk



Ultraview

PCF

LER

FMT SX

LER



10 X 50

12 x 50

15 X 70

16 x 70

20x80

NOTES



Manufacturer's Data


FOV degrees published

6.5°

4.2°

4.4°

4.0°

3.5°

published

FOV Ft at 1000Yd.

341

221

231

210

183

Tan (FOV)x3000



Field Measured Data


separation in mm

M5 fld

46

Aql B

34

Per

35

Per

34

UMi1

26

view 1

as measured in

Serp

49

Del

34

Per

36

Per

33

Del2

25.25+

view 2

SkyAtlas 2000.0

Lyra

47

Del

33

Del3

35.5

Dra4

33.5-

Del2+

26

view 3

10 degrees = 82mm

Lyra

49

Del

35

Gem4

<36

Tau2

32.75+

Dra5

<27

view 4


Per

49

Cas

33.5

UMa2

<36.5-

Her4

33.25-

Dra5-

26.5

view 5


Dra 4

33.5+

Gem2+

35

Gem1

32.75

Tau1

26.25-

view 6


Her 4

33.25+

Gem$-

35

view 7

Use Max measure mm

49

34

35.5

33

26.5

max mm viewed





Actual TFOV

5.98°

4.15°

4.33°

4.02°

3.23°

measured

FOV Ft at 1000Yd.

313

217

227

211

169

Tan(FOV)x3000

Area of View sq degrees

28.0

13.5

14.7

12.7

8.2

pi(FOV/2)^2



AFOV eyepiece

60

50

65

64

65

in degrees

Results Max FOV Tests

The Fujinon 16x70s actually field measured 4.0+°, a true field of view that agrees with the manufacturers specifications. The Oberwerk 15x70s produced a measured result that is just below the manufacturers specifications. The Oberwerk 20x80s field measured about 10% less than specified and the Pentax were nearly at the specified mark.

From the smallest fov, the Oberwerk 20x80s at 3.2°, to the largest fov, the Oberwerk 15x70s at 4.3°, there is a difference of 75% more area of sky in the view. Area = Pi r2 and is listed for each binocular in the data table.

All the binoculars showed a measurable degradation of the image at some distance out from the center of the FOV. The distance out from center varied for each and loss of this useable area will be discussed under the topic of sharpness across the field. Some of the binoculars reduced the total FOV delivered to the eye due to eye relief, depending on whether used with or without glasses.

Conclusions - Object Finding

Although my 35s show a huge chunk of sky and are very nice for general viewing of constellations and sky background, the magnification is not enough for finding a full range of objects.

Within this group tested, the most versatile binoculars for finding objects are the Oberwerk 15x70s. They have a 4.3° FOV, good enough to orient in perspective and the 15x magnification is more than needed to see many types of faint objects. By far, my most versatile binoculars for finding objects are the Orion Ultraview 10x50s. They have a wide enough 6.0° FOV to orient sufficient area in perspective and the 10x magnification is satisfactory to see many faint objects including some galaxies, globular clusters, open clusters, nebulae and many fairly close double stars. The 10x50s provide a comfortable rendition of sky as compared to the scale and magnitude limit of SkyAtlas 2000.0, reaching stars just beyond the atlas.

The 15x magnification made finding small objects easier. Very small objects such as M57, the Ring Nebula, which appeared almost star-like at both 15x and 16x, would be difficult to identify if you didn t already know exact location based on star chart reference. The 70s are much more like using big 50s than like using small 80s.

The Pentax 12x50s have a 4.2° fov. The dropped barrel shape of the Pentax makes it difficult to sight over the top of the binocular and end up in the vicinity of the object. Even with a fov of 4+°, I have a difficult time finding objects with the Pentax 12x50. I have never successfully pointed the Pentax and found anything without panning around for a while. Fujinon 16x70s had almost the same FOV, 4.0°, still not too small and I found them easy to use.

The Nikon Sky and Earth 10x50 Kestrel are the same shape as the Pentax and despite their 5.0° fov I had the same difficulty with pointing them. The dropped barrel shape has me pointing them always several degrees off from the correct spot. The Oberwerk 15x70 are easy. They are like the point and shoot of binoculars. Almost every time I point them in the direction of an object, I look thru and it is in the fov.

The 3.2° Max FOV in the Oberwerk 20x80s was very noticeably smaller. It made it a little more difficult to acquire an area of the sky for orienting to charts. I also found the 80s to be too small a FOV when viewing star fields around the Cygnus Milky Way. Image scale in the 20x80s did improve the viewing of some small close groups.

Examples of Maximum True Field of View

10x50 1 just barely encompasses the parallelogram of Lyra

  • 6.0° 2 just gets Orion s Belt and the entire Sword at once
  • 3 can comfortably view the Hyades with surrounding perspective
  • 4 just barely sees the entire head of Draco

15x70 1 Orion, from the lowest belt star to include all of the sword

  • 4.3° 2 can comfortably view the Hyades but with little perspective
  • 12x50 1 across the bottom of the keystone of Hercules
  • 4.2° 2 across the south side of Draco s Head

16x70 1 Aldebaran to the point of the Hyades with just a bit to spare

  • 4.0° 2 just gets M36 to M37 in the same fov
  • 3 across the bottom of the keystone of Hercules
  • 4 The Beehive, M44 fits very nicely
20x80 1 exactly across the opening of the V of the Hyades
  • 3.2° 2 fits easily Orion Belt stars only or Orion s Sword only
  • 3 the body only of Delphinus the dolphin, no tail
  • 4 The beehive, M44 nearly fills the entire FOV

Fields for Checking MAX TFOV

These are a few of the fields I used to check the Max Tfov of my various binoculars. You can refer to my article “How to Measure TFOV of Binoculars”for a broader list of guide-points for measuring field of view. http://www.cloudynights.com/howto/bino-measure.htm





FIELDS FOR BINOCULAR MAX TFOV



FOV °

point A

point B















UMi 1

3.2

B 7 UMi

y 13 UMi

front of the cup of Little Dipper








UMa 1

5.4

a 50 UMa

B 48 UMa

front of the cup of BIG Dipper








Dra 1

2.9

B 23 Dra

v 24 Dra

west side of Draco head shortest


Dra 2

3.4

v 24 Dra

z 32 Dra

north side of Draco head


Dra 3

5.4

z 32 Dra

y 33 Dra

east side of Draco head longest


Dra 4

4.1

B 23 Dra

y 33 Dra

south side of Draco head


Dra 5

3.3

B 23 Dra

30 Dra

further out south side of head








Tau 1

3.2

a 87 Tau

e 74 Tau

Aldebaran across the open V, the Hyades)

this is just a hair too wide for the Oberwerk 20x80s.

Tau 2

4.0

a 87 Tau

y 54 Tau

Aldebaran to the point of the V the Hyades)

The 16x70 Fujinons have a little room to spare on this.







Ori 1

2.8

z 50 Ori

d 34 Ori

Orion s Belt stars

These fit my Oberwerk 20x80s with room to spare.







Gem 4

4.4

1 Gem

13 Gem

the whole foot

15x70s a hair less than this = 35 = 4.3°







CMi 1

3.3

a 10 CMi

n 5 CMi

Procyon to companion


CMi 2

4.3

a 10 CMi

B 3 CMi

Procyon to Gomeisa

16x70 just <<, 12x50 just <, 15x70 just >







Her 2

6.6

pi 67 Her

e 58 Her

east side of the Keystone of Hercules


Her 4

4.1

e 58 Her

z 40 Her

bottom of the Keystone of Hercules








Cyg 1

6.1

a Cyg

y Cyg

Deneb to Sadr top to center of cross








Del 1

2.7

y 12 Del

B 6 Del

body of Dolphin


Del 2

3.1

y 12 Del

z 4 Del

body of Dolphin + #4

Near maximum view recorded with 20x80 Oberwerks

Del 3

4.4

y 12 Del

n 3 Del

body of Dolphin + #3 in the tail

maximum view recorded with 15x70 Oberwerks

Apparent Field of View

Based on the formula TFOV = AFOV / Mag., if we know TFOV, then we can calculate AFOV = TFOV x Mag. This provides the resultant apparent field of view of the eye lens. The Pentax 12x50s have a 50° Afov, similar to a plössl eyepiece. I ve now tested twelve different binoculars. These Pentax along with Nikon s Kestrel 10x50 and Orion Giant 16x80 all have an Afov of 50° to 53°. All the other binoculars have an Afov of 60° to 66°, similar to a SWA, Konig. There is a big difference to the eyes when looking into binocs of 50° next to binocs of 66°.

The Apparent Field of View in the eyepieces has a tremendous affect on the visual experience. Telescope eyepiece types are rated by their Apparent fov, Orthos 42°-46°, Plössls 50°-52°, Superwides, Konigs and Panoptics 60°-68°, Utrawides and Naglers 82°-84°. This has a significant impact to the eye on the visual experience thru the lens. As an example, the Oberwerk 15x70s have an Afov of 4.4°x15 = 66°. The Pentax 12x50s have an Afov of 4.2°x12 = 50°. When you look thru the Pentax you see almost the same amount of sky (4.2°) as seen in the Oberwerk (4.4°), BUT it's like looking down a long tunnel. The Pentax 12x50 has a lot of black barrel surrounding the view to the eye. The Oberwerk is framed very nicely and the eye is presented with a clear wide visual experience, almost no barrel seen. To me this is much more pleasant. It's like not having binoculars in front of you but just seeing a magnified image of the sky. Not so with the Pentax Afov of 50°, which is like seeing the image at the end of a long tube.

TYPES OF VIEWING

Binoculars need to satisfy multiple requirements. Not only must they have On-Axis sharpness for maximum resolution, but also they must have a sharp wide area of view to encompass large objects such as cluster groups, large galaxies or extended nebula. In addition, sometimes it is desirable to frame large objects or areas with surrounding sky to view the objects in perspective. These uses all require varying levels of sharpness across an increasingly wider field of view.

This is my approach to identifying how binoculars are used. The next step will be to measure the sharpness across the field of view to determine how much of the binocular is really usable for the desired type of viewing.

Finding Limit

We have established the maximum Field of View (TFOV). This Limit is that usually not all of the FOV is useable. For finding, generally as large a field of view as possible is needed to view the sky in perspective. Finding usually involves orienting to a chart. We are not observing for detail here, so the zone of view out to and including the Poor zone can be considered usable. In effect, this is reducing the overall field of view of the binoculars and indicating that only a certain percentage of the Max FOV is useable. The area of view considered Bad is not considered useable.

Wide Detail Viewing

This viewing limit uses the maximum wide fairly sharp area of the FOV. In most cases the area including the Fair zone can be considered the Wide Detail Viewing Limit. This type of viewing is used for star fields, cluster groups and galaxy clusters. The objects viewed in the Fair zone are not well defined. The Fair zone is useable only for viewing the outer edges of detail for wide field objects.

Best Detail Viewing

This viewing uses the maximum clearly sharp area of the FOV. This would be only the area considered Excellent or Good. This type of viewing is best used for large extended objects, cluster groups and galaxy clusters that would all require a sharp resolution at the outer limits of the objects being viewed.

Sharp On-Axis Detailed Observation

This is detailed observation of generally small objects using the center of the FOV. All of the binoculars had at least 40% to 50% of the FOV considered Excellent to Good. This is more than enough for double stars, most galaxies, globular clusters and many open clusters.

SHARPNESS - USEABLE FIELD OF VIEW

Sharpness

Several star fields were used to test for sharpness across the entire FOV. The binoculars were moved back and forth slightly so that star images that were focused clearly in the center were observed at various zones out to the extreme edge of the FOV. Observations were recorded several times. Sharpness is recorded as Excellent, Good, Fair, Poor or Bad. Bad results are areas of the viewable field where the star images are extremely distorted or enlarged blobs.

Poor results are areas where the images are distorted or enlarged, but might not be distracting at the outer fringes of surrounding perspective. The area out to and including Poor is considered the useable area for finding and perspective. A smaller area, only out to and including the Fair zone is considered the limit useable for viewing the outer edges of detail for wide field objects. Once the field enters the Poor zone images become unsatisfactory for viewing objects. Stars are flared, bloated or distorted beyond a desirable limit. As an example of the transition from Fair to Poor, Albierio - B Cygnus, a wide double at 34”, becomes too blurred to separate into it s two components.

Recently, I ve been testing sharpness of field by testing a 22" double across the field. All the binoculars in this comparison handle those more stringent criteria further out from the center than the Orion 16x80 did with the 34" double.

Distorted images in the Poor or Bad zones could not be refocused to improve image. No detail, even the outer edges of wide clusters, could be satisfactorily viewed in the Poor zone or beyond. The faintest stars resolvable are not seen in the Poor zone.

Sharpness

Orion

Pentax

Oberwerk

Fujinon

Oberwerk


across the FOV

Ultraview

PCF III

LER

FMT SX

Giant



10 X 50

12 x 50

15 X 70

16 x 70

20 x 80


% of distance center to edge








22"



30"



22"



40 and less

E

E

E

E

E

E

E

E

E


E

E

E

E

E


40 to 50

G

G

G

E

E

E

E

E

E


E

E

E

E

E


50 to 60

G

G

G

E

E

E

G

VG

G


E

E

E

E

E


60 to 70

F

G

F

G

G

G

G

G

G


G

G

G

VG

G


70 to 80

P

F

P

G

F

G

F

F

F


G

G

F

G

F


80 and greater

B

P

B

F

F

F

P

P

P


F

F

P

F

P


90 and greater




P

P

P



P


P

P

P

P

P



















Maximum FOV degrees



6.0



4.1



4.3



4.0



3.2

measured

Max. Area of Sky



28sq°



13.5sq°



15sq°



13.5sq°



8sq*

pi(FOV/2)^2


















FINDING LIMIT

















Usable Limit = Poor



80%



90%



90%



90%



90%

% to center

Finding Limit FOV degrees



4.8



3.7



3.9



3.6



2.9

% x max

Usable Finding Area sq dgr



18



11



12



10



7

pi(FOV/2)^2


















WIDE DETAIL VIEWING LIMIT

















Usable Limit = Fair



70%



90%



80%



90%



80%

% to center

Detail Limit FOV degrees



4.2



3.7



3.5



3.6



2.6

% x max

Usable Detail Area sq dgr



14



11



9



10



5

pi(FOV/2)^2


















BEST DETAIL VIEWING LIMIT

















Usable Limit = Good



60%



70%



70%



80%



70%

% to center

Detail Limit FOV degrees



3.6



2.9



3.0



3.2



2.3

% x max

Usable Detail Area sq dgr



10



7



7



8



4

pi(FOV/2)^2

Results of Sharpness Test

The 20x80s were rated Poor between 80% and 90% out from the center. Images between 90% out became distorted. The narrow 3.2 Max FOV of the 80s is only reduced to a finding limit of 2.9° out to and including the Poor zone. The Fair zone, at 80% out from the center, reduces the usable wide detail-viewing limit to 2.6°. That is more than enough of a FOV for the 80s to capture the area of the Pleiades with no distortion. Orion s Sword, the nebula with the surrounding star clusters, would barely fit and Praesepe would be comfortably bordered.

The Fujinon 16x70 has a very sharp image, and it stays sharp out to 80-90% of the fov. Stars are brought to a finer image point than any other binocular. In the Fujinon 16x70, images were Poor only beyond 90% out. The 4.0° Max FOV is reduced to a finding limit of 3.6° out to and including the Poor zone. (This encompasses 75%% of the area of the fov). The Fair zone, at 90% out from the center, maintained the usable wide detail-viewing limit at 3.6°.

Although the 15x70s were Poor at 80% out from center, they weren t Bad until near the very edge. Therefore, the Poor zone was considered to extend out to 90%. The 4.3° MaxFOV of the 70s is reduced to a finding limit of 3.8° out to and including the Poor zone. (This encompasses 80% of the area of the fov). The Fair zone, at 80% out from the center, reduces the usable wide detail-viewing limit to 3.4°. That is sufficient FOV for the 70s to capture all of Orion s belt stars in one view and more than enough to view Orion s Sword. A good portion of the open clusters surrounding the Rosette in Monoceros would fit comfortably. This 3.4° is about the smallest FOV that could provide a hope for seeing an object like the extended North American Nebula all in one view.

In the Pentax 12x50, images were Poor only beyond 90% out. The 4.2° Max FOV is reduced to a finding limit of 3.7° out to and including the Poor zone. (This encompasses over 80% of the area of the fov). The Fair zone, at 90% out from the center, resulted in a usable wide detail-viewing limit at 3.7°. That is large enough of a FOV for the 12x50s to encompass all of the Coma cluster, Mel 111, or easily M24 the Small Sagittarius Star Cloud or easily the M20-M8 the Trifid-Lagoon Nebula area.

Orion Ultraview 10x50s have a 6° measured fov but can't begin to compare to the sharpness of the Pentax. Orion s are sharp only out to 60% of field, between 70-80% out the image becomes un-useable, leaving only about a 4.2° clean wide field view. In the Orions favor, the wider finding limit does make it easier to initially find objects and at 32oz. they can easily be handheld.

Conclusions - Opinions of Sharpness

The Pentax 12x50 has an incredibly sharp image, and it stays sharp out to 80-90% of the fov. Stars are brought to a finer image point than seen in the Oberwerk 15x70s.

The Oberwerk 15x70s had a greater % of the total FOV usable for viewing than any other binocular in this comparison. They are very nearly equaled by the smaller Pentax 12x50s.

All four of the big binoculars had 90% of the fov usable for finding. Both the Pentax and the Fujinon had more than 80% of the fov usable for wide-field viewing, the Oberwerks just a little less.

The Orion 10x50s provide almost two to three times the area of the 70s, but at a significant reduction in magnification. All the other binoculars provided nearly twice the field or more for viewing than the 20x80s. The Pentax have the slight edge in the size of sharp field for viewing.

Based on these observations, the 16x70 Fujinon are by far the best performers as pertains to sharp field of view and magnification for detailed viewing. The 10x50s offer a good combination of 4°+ sharp field of view at a decent 10x power, which is sufficient for a great many objects. However the 15x or 16x magnification helps bring out so many more faint objects that the magnification adds a significant advantage.

OBJECT VIEWING

Wide Detail Viewing

Open Cluster Groups like the area around Y Cygnus or the area around 2244 Mon - the Rosette, very large Open Clusters such as the Hyades or Mel 111 - Berenices Hair and small constellations such as Sagitta, Lyra or Delphinus require a fairly sharp view over a very wide area if all stars are to be seen clearly. These areas might be viewed using the FOV out to the Fair limit considered the Detail Viewing Limit. Other large area objects include M24 Small Sagitarius Star Cloud, Alpha Perseus Association, Cassiopeia clusters near M103

Best Detail Viewing

Some objects required a considerable degree of sharpness across a moderately wide FOV to be seen in sharp detail out to the edges. These objects generally are viewed entirely within the Good area of the FOV, considered the Best Detail Viewing Limit. Examples of objects in this class are galaxy groups such as M81-M82, large open clusters like NGC 752, the Pleiades, IC4665 in Oph and the Double Cluster in Perseus. Other best detail objects include very large galaxies such as M33, M101 and M31, areas like Orion s Sword and objects like the California Nebula or the North American Nebula..

Brightness gradients have a significant impact on our ability to view extended objects. In the Best Detail Viewing area, light gradients across diffuse objects are more clearly defined. Contrast improves the view within the area. Within the Fair zone sharpness already distorts image enough that it reduces the ability to discern diffuse objects. In the Best Detail Viewing, the image fidelity is sharp enough to differentiate diffuse light from dark sky background or even from brighter diffuse light. This allows seeing faint extended objects that would not otherwise be seen and allows resolving point sources in the diffuse glow of scattered light made up of dense clusters.

On Axis Detail Viewing

Many objects require only a very sharp On-Axis view to be seen in detail. Little surrounding perspective is required, although some perspective is always helpful. These objects are generally viewed entirely within the Excellent area of the FOV. Examples of objects in this class are individual galaxies, globular clusters such as M3, M5 and M13, planetary and diffuse nebula and double stars. Nice Binocular doubles range all the way from easy vDra/62”to Alberio down to close.

Objects in the Viewing List

Many nights were spent viewing objects of interest. Time was spent on various combinations of double stars to test for the limit of each binocular. Examples of galaxies, diffuse nebula, globular and open clusters and planetary nebula are included in the viewing. In some examples, resolved stars are counted to test comparative magnitude limits. Nearly all of these objects are small enough to be viewed in the center of the FOV. Several very wide open clusters were included in the viewing list and attempts were made to see some large nebula that could only be fully encompassed by a sharp FOV over 3*.

I found M33 in binocs before I ever found it in a scope. Rarely do I go out now without taking a pair of binocs on a tripod to have as a quick look. I've logged probably hundreds of objects in my various binocs. You can't see M16-M17-M18 all in one field of view in a scope, you'd need a 4* fov. I suppose you could take a fast f5 or f6 wide field scope and pop in a long wide field eyepiece to get an incredible low power wide field view. But that's pretty much what we do with binoculars, and we do it with two eyes.

I've been spending three out of four nights with binoculars lately, so my list here is those objects I seem to always return to for another view with the binoculars. These views are with 15x70's 16x70s and 20x80's and both late at night and early morning.

The Double Cluster - stunning in 20x80s, extended resolving in both clusters.

In the 4.3* fov of the 15x70's, the double cluster is seen in concert with St2.

Orion s Sword, M42 and the Trapezium. In the 16x70s AB/8.7”is elongated and not split, but C, D, AB appear clearly as three components of the famous quad. Surrounded by the gaseous glow of M42, this may be the most visually stunning multi-component system in the galaxy.

M31, the Andromeda Galaxy. I see extension of about 1° to 1 ½° in the 15x70s and 1 ½° to 2° in the 20x80s. M32 is always seen in combination and M110 sometimes seen in combination.

Saturn - In the 15x70s the rings are just seen at the edge with a black space separating them from the disk. In the 20x80s, the rings stand out clearly from the disk. Titan does its dance if you watch from night to night.

M45, The Pleiades. There are very faint stars at the outskirts of a 4° fov adding to the faint double and a triple in the central area in addition to another 50 stars seen in the binocs.

Splitting doubles with binocs

I recently spent a month using binoculars at 10x, 15x, 16x and 20x to find the limit of each binocular by testing the splitting of doubles. I found that at 10x50 the limit was about 20", but at 16x80 or 15x70 the limit was 13" or 14". I had my greatest success with doubles that were close in magnitude. Where the magnitude varied a lot (+2.5) between the primary and the secondary, the difficulty factor increased the limit by 5" to 10".

With my 10x50 Ultraviews I have split doubles at 22" Alya-Ser, 21" 61-Oph and 20" 24-Com. With the 12x50s I split 100-Her 5.9-6.0/14.2” and E2738 Del 6.6-8.7/14.9” appeared elongated. With 15x70s successful targets were E(sigma)2474 Lyr at 16.2" and 100Her at 14.2". With 16x80s I split E2474, 100Her and also E2470 at 13.4".

Recently I attempted (gamma) h Del 9.6” with my 15x70s. I recorded that I suspected elongation in this 4.5-5.5/9.6" pair. Although I did not see a clean split, my notes confirmed that I had the correct PA of the secondary. I have seen this double cleanly split with the Oberwerk 20x80s and also with the Fujinon 16x70s.

Just as in a scope I have a much more difficult time splitting a pair if there is a wide range, say 2.5+/-, in the magnitudes. Also I have a hard time if the primary is very bright. I've had a hard time splitting Mizar 2.3-4.0/14.4" with the Oberwerk 15x70s, but was successful with the Fujinon 16x70s and the Oberwerk 20x80s. With Orion 16x80s I was only able to confirm elongation with split strongly suspected.

Top 10 doubles for Binoculars

Some of these you need 15x to see

  • Alcor Mizar show kids, ask can they see it without binocs
  • 24 Draco very wide and even
  • B Cygnus Alberio color shows well
  • Alya Serpens Cauda never leave the area without a look
  • 61 Ophiucus
  • Kappa Boo
  • 100 Herc even at 14" this one is easy to see @ 12x
  • Gamma Delphinus need 16x to 20x
  • d1d2 Taurus
  • Trapezium ABCD (AB not always seen )


Astronomy is not just viewing, it's seeing. Keep a record of what you see.

DESIRED ATTRIBUTES

Brightness

Brightness was sufficient in the 70s to see a star like core in eg M81 with a broad surrounding glow, a pear shaped eg M51 with the south member larger and brighter, and a bright core on gc M13 with fainter circular outer extension. The 80s showed M13 with a large bright core with a slightly dimmer outer glow and M51 had slightly more defined shape with the south member more prominent. With both binoculars a number of globular clusters were observed including M3 and M13 as the brightest examples, M5 and M10 that showed a core and fainter examples M92, M71, and M12. Last summer, my 10x50s gave a view of M22 that appeared so large and bright it seemed like an open cluster.

Contrast

I did notice considerably darker background sky in all of the 70s and 80s when performing the aperture mask test. I attributed the greater ability of the higher magnifications to see fainter stars to the increased contrast associated with higher power.

A few years ago, my 10x50s provided my first ever view of eg M33, an object I had searched for many times with the telescope to no avail. M33 stood out from the background better in the Fujinon 16x70s than any of the other binocular. Although all the binocs could see the Pinwheel, the contrast in the Fujinons made it much easier to see the faint diffuse light of M33. The 15x70s have provided me with an averted sighting of another elusive galaxy, M101. The 20x80s brought out the fainter south component of M51. Using the Fujinons, I found M31 in daylight about 20min before darkness.

Saturn s rings are separated from the disk in all three larger binocs. The Trapezium is resolved to 3 stars in all three binocs. A-B at 8.7”is usually not resolved, but several times did appear elongated. On one rare occasion with the help of 20x mag., AB was resolved. This is one instance were I think magnification provided the benefit.

On small dense clusters that often just look like a glow in binoculars, I saw a real difference. The Fujinon contrast and resolution makes it easier to pick out the fainter stars and pick out the stars embedded in the glow of many in the small tight clusters. In a second test on M36, Ob15x70 saw 4*, Ob 20x80 saw 4*, Fuji 16x70 saw 8*. On M37, the Oberwerk 15x70s saw 3* while the Fujinon 16x70s saw 8*.

Point Source Resolution

The 20x80s split the double star (gamma) y 12 Delphinus 4.5-5.5/9.6”. The 15x70s show y Del as elongated with proper orientation noticeable. The Oberwerk 20x80s and the Fujinon 16x70s show Sigma 953 Mon elongated with proper orientation noticeable. E953 Mon is 7.2-7.7/7.1”. I noticed this with the Fujinon when I was viewing the Christmas tree cluster. At the time, it wasn t even in the center of the fov. These same three binoculars regularly resolve 3 stars of the Trapezium in M42 and on occasion the 4th was suspected in the Fujinon 16x70s and Oberwerk 20x80s. Finally, on an exceptionally still clear night, the AB component at 8.7”was resolved in the 20x80s at full aperture and also at 20x50. The 20x provided just a bit more magnification that allowed easier splitting of close doubles than could be accomplished with the 16x70s or the 15x70s.

The Fujinon and the Pentax can be focused to a finer point of light than either of the Oberwerks.

Resolution by Contrast

I had ample opportunity to view Saturn it is so well placed. Separation of the rings from the disk was observed on many occasions with the 15x70s, 16x70s and 20x80s. Separation between the disk and the rings at the widest point is only about 6” to 8”.

Stars resolved in dense open clusters increased as contrast and resolution improved. On one occasion, I reported seeing M36 as 6* in the 15x70s, but I counted 13* with the Fujinon 16x70s. Sky Catalogue 2000 list M36 as 60* with 12 diameter. Burnham s states that M36 has 15* ranging from mag8.8 to mag10.6. Read what O Meara wrote about M36 and M37, how he likens both to globular clusters.

Out of Focus Star Test

This is where I realized the affects of the astigmatism in my left eye. I like to view without my glasses and when doing the star test without my glasses I noticed considerable astigmatism in the left lens. I put my glasses back on, performed the out of focus test again and found both the 70 s and 80 s have pretty nicely rounded out of focus images both inside and outside of focus.

Color

In daylight views of a well-lit bright white snow covered scene, the Fujinon provided almost an exact rendition of the scene color. I would refer to it as white with good color contrast. The Orion Ultraview gave nearly the same color rendition. The Pentax 12x50 showed color as white/beige. Both Oberwerks were very much beige or almost yellow/tan, giving a noticeable off-color to the very white snow.

ABERRATIONS

These are the details on just those aberrations that I made a note of during my observations. The most notable aberration, sharpness across the field of view, which is not really an aberration but a limitation of the quality of lens construction, has already been discussed at length.

Rectilinear Distortion curvature

In my last article I referred to Pincushion as Rectilinear Distortion. It has the effect of causing objects to curve outward from each other as you near the edge of field. Same thing, different term. It is easily noticed on straight lines, poles, building sides. It has almost no apparent disadvantage to me in dark sky viewing.

The Orion 50s had moderate curvature. Vertically straight objects curved outwards on both sides of the center of view, but magnification wasn t high enough to make it distracting. Curvature could not be seen in the Pentax 12x50s. The Fujinon 16x70s had minor curvature. It was not at all distracting. Both the Oberwerk exhibited considerably more curvature, more so in the 70s. It could be noticed at 20% out from center and at 50% out from center straight vertical objects were significantly curved outward to the point of distraction. This defect was not noticed during any nighttime dark sky viewing. Its affect on deep sky objects would be to move the outer stars in the field slightly from the true picture location. I think an easy test for this would be to put a nice wide double like v Dra 62" into the field of view and move it around to check the apparent separation. If pincushion is present it will show closer to the edges.

Chromatic Aberration

The 15x70s showed a very unpleasant broad yellow border around the leading edge of the half moon. This would sometimes disappear as I moved my eye around the field. When I was testing them with a 50mm aperture mask over the objectives, very little color fringing was apparent. With the 50mm mask, the binoculars were changed from f4.8 to f6.7.

Little or no apparent color fringing was seen on bright stars. I viewed Arcturus, Spica, Deneb, Altair, Vega, Aldebaran, Capella, Betelgeuse, Bellatrix, Rigel, Castor, Pollux and Procyon. The 15x70s seemed to show a very slight color fringe only on Vega.

The 15x70s, 16x70s and the 20x80s show apparent blue color fringing on Venus at 40% illumination. I was able to move my eyes off axis in the 15x and 16x to produce false color on the fully illuminated edge. The same effect appeared in both, blue at one edge of the exit pupil, yellow on the other. When I moved my eyes back on axis, the color disappeared in both. All three show blue fringe to one side of Jupiter.

In a bright daylight snow-covered view, the Fujinon 16x70s off-axis view was terrible. There was blue fringe everywhere. With my eyes back on axis the blue fringe disappeared. The Fujinon actually provided the best true color rendition of the scene.

Astigmatism

None noticeably present. I was pleased to find that the astigmatism I originally thought to be a defect in my lens was being induced by my eyes. Refer to the notes in Star Test above.

Field Flatness

I have tested for sharpness across the field. I have not tested for field flatness. Flatness could be skewing the results in the sharpness test. How much of the distortion is caused by sharpness falloff and how much by lack of field flatness?

Moving a star around in the field of view and attempting to refocus to a fine pinpoint image at various positions can test for Field flatness. If sharpness of the test star falls off and it cannot be refocused, the problem is due to sharpness across the lens system. If sharpness of the test star falls off but it can be refocused at some of the test points say at 40%, 60% or 80% out from center, the problem is due to a lack of field flatness of the lens system. If a lack of field flatness is discovered, there may be some limited ability to average focusing of star fields so stars towards the edge are not so severely out of focus. This would slightly compromise the on-axis sharpness of the image. CREDITS

Thanks to these people, whose equipment evaluations, informative writings and help I found invaluable. Note that many of these references pertain to telescope data. The relevance of optics data is not lost as it pertains to binoculars.

Terence Dickinson & Alan Dyer, “The Backyard Astronomer s Guide”, Firefly Books, 1998; required reading on attributes of optical systems, general astronomy must know information.

J. W. Seyfried, “Choosing, Using & Repairing Binoculars”, University Optics, 1995; for information on how binoculars work and especially collimation.

Stephen James O Meara, “Deep Sky Companions, The Messier Objects”, Cambridge University Press, 1998; outstanding descriptions of the Messier objects.

C. R. Kitchin, “Telescopes and Techniques”Springer-Verlag, 1995; for textbook explanations of various aspects of optics and telescopes.

Michael A. Seeds, “Foundations of Astronomy”, Wadsworth Publishing Company, 1997; for textbook explanations of various aspects of optics.

Alan Adler “The REAL Scoop on Binoculars”, http://www.weatherman.com/ for notes on the relative importance of Magnification, Object Finding and Object Viewing;

Jay Freeman http://observers.org/beginner/ for his articles Astronomical Telescope Eyepieces: A Discussion for the Beginner, and Cosmic Bird Watching: Visual Observation of the Deep Sky.

And to these Cloudy Nights authors, in addition to all the others whose articles appear on this website.

Michael Hosea “Notes on Eyepiece Evaluation”for the notes on sharpness, brightness and contrast; Allister St. Claire for his “Editorial Commentary” and for providing the forum where we all have the opportunity to share valuable information.

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