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Comparing GO22x85, WO22x70, Tak22x60

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#1 EdZ

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Posted 21 April 2009 - 09:58 PM

Well, there aren't many binoculars in those sizes, so you might be already guessing at which I intend to compare. But don't go getting all anxious, you know how long it can take me to get through a series of tests comparing binoculars. A few months wouldn't be too short a time and may not be long enough.

For starters, I can give you some of the preliminaries I've measured.

Garrett Optical 22x85 Signature
full aperture appears to be 84mm
36mm or 43% center of aperture provides 100% illumination of exit pupil
when set to distance exit pupil = 3.94mm
magnification therefore is about 21.3x, call it 21x
21x84 with 43% of aperture providing 100% illumination

William Optic 22x70 Apochromat
full aperture appears to be 70mm, may be 71mm
35mm or 50% center of aperture provides 100% illumination of exit pupil
when set to distance exit pupil = 3.36mm
magnification therefore is about 21.1x or 20.8x, call it 21x
21x70 with 50% of aperture providing 100% illumination

Takahashi Astronomer 22x60
full aperture appears to be 60mm
50mm or 83% center of aperture provides 100% illumination of exit pupil
when set to distance exit pupil = 2.72mm
magnification therefore is about 22.05x call it 22x
22x60 with 83% of aperture providing 100% illumination

Well, the most outstanding measure of the above is the percent illumination of the Tak 22x60. I've now measured about 70 binoculars for illumination. Only 10 of them are over 40%. Only 4 of those reach 50%, two Fujinons, a Pentax and this WO 22x70. BUT only one exceeds 50%, the Tak Astronomer. And it shows that an astounding 83% of the center of aperture provides 100% illumination to the exit pupil. Now follow me here, I'll explain what this means.

Both the GO 22x85 and the WO 22x70 have 35/36mm of the center of the lens that is providing 100% illumination of the exit pupil. Then they both taper from 100% to zero at the edges. Assuming illumination in both is a constant slope of dropoff from full to zero at the edge, then the GO Signature will have slightly more total light delivered to the exit pupil, simply by nature of the wider aperture. But how much more? Surprisingly, not as much as you might think.

If you break down the light into concentric rings so you can calculate how much light from each ring, you can approximate how much light is delivered into the exit pupil. The central area of 35mm dia is full 100% lit. But, for example, take the 10mm wide ring between 45mm and 55mm diameter. 55sqrd minus 45sqrd leaves us with a ring of 3025-2025=1000sqmm. For sake of our example lets say that ring delivers 70% of its light. Therefore it effectively delivers about 1000sqmm x 70% = 700sqmm of light. This continues to decrease in each concentric ring further out until it drops to zero at the edges. You can do this for each concentric ring and approximate the total light in the exit pupil by adding up the subtotals for each individual ring. Two more things about this; you can simply take the entire area outside the central circle, and say that area provides 50% illumination; but the drop off slope is not constant.

The GO22x85 has almost 50% greater light gathering area than the WO22x70. But when you calculate total light, because the GO22x85 has a smaller percent of full illumination, the GO22x85 delivers only 37% more light than the WO22x70.

Now compare the WO22x70 to the Tak22x60. Well, the WO22x70 has 36% greater light gathering area more than the Tak22x60. But the Tak has so much of it's lens area that provides 100% illumination, 83% for the Tak vs 50% for the WO, that the Tak actually delivers the same total amount of light into the exit pupil as the WO22x70.

Now compare the GO22x85 to the Tak22x60. The GO22x85 has 2.0x the light gathering area as the Tak22x60. However, we have already determined the Tak22x60 delivers the same total amount of light into the exit pupil as the WO22x70. And we have already determined the GO22x85 delivers only 37% more light than the WO22x70. Therefore, the GO22x85 delivers only 37% more light into the exit pupil more than the Tak22x60.

The GO22x85 is very good at light delivery. At 40% full illumination, it ranks among the top 10 I've seen.

The WO22x70 has an even more efficient light delivery system. It measures at 50% full illumination, ranking it with 4 of the top 5 binoculars I've seen. I'd consider it excellent.

The Tak Astronomer 22x60 has such an efficient light delivery system that it is acting more like it is an excellent 22x70. It is so efficient that it is a superior 22x60.

edz

I want to thank Peter Staiger for loaning me two fine binoculars so I can make these and more comparisons over the next few months.

#2 pcad

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Posted 21 April 2009 - 10:32 PM

I was rooting for WO to make the very popular 66ED scopes into binoculars since they were demo'd a few years ago. I wasn't going to let the binoculars based on their larger ED scopes slip away. I was able to get them for the NEAF price last year.

I was fortunate to be able to get a used Tak 22x60 earlier this year. I figured that it would hold it's value and I'd be able to see for myself what they are like.

Despite using and enjoying these binoculars I knew I wouldn't do a comparison review justice. I just don't have the time, inclination or ablity to do such a review.

Fortunately Ed doesn't live too far from me and NEAF presented an ideal opportunity to loan them to him.

I have a few suspicions about why they are such different instruments. I'm as interested as anyone here as to what his conclusions will be. He's already found out more about these binos in one day than I have in 6 months.

#3 Mike Hosea

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Posted 21 April 2009 - 10:44 PM

I had not heard of the WO22x70s before this. The pictures on the WO web site look to me like it's based the 22x85 in most respects but adapted to the different lens cells and premium objectives.

#4 EdZ

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Posted 21 April 2009 - 10:49 PM

The back end of the WO22x70 is identical to the back end of the GO22x85. Right up to the first smooth ring on the barrels, right in front of the prisms. In fact, if you cut the binocular right at those smooth rings it would be the Oberwerk Ultra 10x50.

The 70mm objective light cones deliver light better through that back end system.

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#5 BobinKy

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Posted 21 April 2009 - 10:54 PM

EdZ...

For me, this will be a very interesting review, as I use the GO Signature 22x85. I agree with Peter, I have already learned more from your first post than I have on my own during the past 12 months.

#6 pcad

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Posted 21 April 2009 - 10:56 PM

I wouldn't say it's based on the 22x85 so much as it shares the body of the 7x50, 10x50, 10.5x70, 15x70, 15x85, 22x85, 20x110 and the 28x110 binoculars that Oberwerk call Ultras and similar ones sold by a number of others. I belive the front end is from the 70mm Zenithstar ED scope and uses FPL-51. As far as I can tell the eyepieces are those found on the 10x50, 15x70, 22x85 and the 28x110.

#7 BobinKy

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Posted 21 April 2009 - 11:09 PM

EdZ...

Among other nitty gritties--I hope you compare the current price for each of these models.

#8 BobinKy

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Posted 21 April 2009 - 11:12 PM

Peter...

This is getting more interesting by the minute--I also use the WO ZenithStar 70mm ED scope.

#9 EdZ

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Posted 21 April 2009 - 11:36 PM

This first study should also highlight for our readers another important fact about binoculars. All to often discussions of exit pupil are brought up and inevitably it leads to thinking larger exit pupil is brighter. Well, that forces you to assume that all binoculars being compared are of the exact same quality and construction. It does not take into consideration the illumination of the exit pupil. It assumes the percent illumination is the same in every binocular. Well, we know that is not the case.

Look here at the WO22x70 and the Tak22x60. By the standard basic discussion of exit pupil, you would be lead to believe the WO22x70 will be brighter. But the far greater percent illumination of the Tak equalizes the light delivered into the exit pupil. So, here's a case where we have equal power binoculars, the WO22x70 has a 3.3mm exit pupil and the Tak22x60 has a 2.7mm exit pupil, and yet the amount of light delivered to the exit pupil is equal. It's all very much dependant on illumination.

You cannot assume just because a binocular has a larger exit pupil that it will deleiver a brighter image. That forces you to accept that there are never differences in the quality of the delivery system. Again, we know that quiite often is not the case. Trying to assess brightness without determining illumination can be quite misleading.

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#10 Mike Hosea

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Posted 22 April 2009 - 01:32 AM

and yet the amount of light delivered to the exit pupil is equal.


Even after accounting for the difference in AFoV? :poke: It's a bit perilous to speak of the exit pupil as a whole. I take it you mean that the smaller aperture may deliver more light at a given apparent field angle, primarily because the corresponding light cones are less obstructed by limited-size prisms but also possibly due in some measure to transmission differences. If this happens "enough" (I believe you described a Riemann Sum approximation to a certain integral), then the smaller aperture may indeed deliver an overall brighter view.

#11 daniel_h

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Posted 22 April 2009 - 03:48 AM

and the WO is offering free intl shipping -i loved my WO 66 -don't think the wife will agree even though our government just gave us 900 as a recession breaker
Peter, if you have both which one are you keeping :question:

#12 Wes James

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Posted 22 April 2009 - 04:32 AM

Edz-
Wow.... a lot of info- and a lot of posts here. You've obviously been busy! I was really happy to hear that you were going to be reviewing these 3 bino's, as I have the W/O and Tak's both as well, and have recently kind of been interested in the Garrett's. Your technical knowledge and testing methods are unparalleled anywhere... so I look forward with great anticipation to your continued information on this great trio! What an opportunity to have all 3 together at one time.

#13 RichD

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Posted 22 April 2009 - 04:42 AM

Very interesting results. I guessed the Tak's illumination would be highest but i never expected 83%! Takahashi must have taken great care in producing these.

Any observing reports with the WO and the taks edz?

I would love to see a similar review of the 18x70 nikon astrolux. I suspect these would show very high illumination too, owners often talk of brighter views than the fujinon 16x70, despite the smaller exit pupil.

Thanks again Edz

#14 EdZ

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Posted 22 April 2009 - 05:04 AM

and yet the amount of light delivered to the exit pupil is equal.


Even after accounting for the difference in AFoV? :poke: It's a bit perilous to speak of the exit pupil as a whole. I take it you mean that the smaller aperture may deliver more light at a given apparent field angle, primarily because the corresponding light cones are less obstructed by limited-size prisms but also possibly due in some measure to transmission differences. If this happens "enough" (I believe you described a Riemann Sum approximation to a certain integral), then the smaller aperture may indeed deliver an overall brighter view.


Perhaps I use the wrong words there. What I am measuring is illumination. How well does any small sample point on the aperture illuminate the exit pupil. So I should say the amount of illumination in the exit pupil. I'm making the inference above that any test point on the objective that delivers light to only 50% of the exit pupil is delivering only 50% the amount of light entering at the point, and so on. Of course, this is caused by internal vignette, where ever it occurs, probably mostly, if not all, in the prisms.

This measure of light delivered into the exit pupil is captured by moving a small laser over the surface. The test gives the result of a 2mm section of the binoc aperture. By moving the laser around, I test various positions of the binoc aperture. See the 22x85 thread where I show pictures of the procedure. Basically this is a 1-2mm aperture target laser tested over various positions on the surface of the full binocular aperture. It seems really to not be dependant on fov. For instance, I've tested seven different 20x80 all with the same method. Most of those 20x80s have nearly identical fov (all near 60-63 Afov), yet they produce illumination that ranges from <5% fully illuminated to 40%. Another example is three binoculars with Afov of 43-46°, yet they show illuminations of 25%, 50% and 83%.

Mike, I think your math skills are a bit beyond mine, but I think if I interpret correctly, I'd say yes I am essentially summing (integrating?) the result from a large number of samples taken over the surface.


Here's a method I use for measuring effective aperture. It is also used to determine the percent of the objective that provides 100% illumination. It's really quite simple. You can refer to the post (EdZ 02/27/09 06:07 PM) in this thread to read the description of what can be measured or just follow further down to the picture presentation to see how its done.
Measuring Effective Aperture with a Collimating Laser Target.

edz



edz

#15 EdZ

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Posted 22 April 2009 - 05:21 AM

Any observing reports with the WO and the taks edz?


as I mentioned in the first post, even the simple stuff, getting out to veiw side-by-side, will all take some time.

edz

#16 Jim Rosenstock

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Posted 22 April 2009 - 06:04 AM

EdZ...

Among other nitty gritties--I hope you compare the current price for each of these models.


A quick web search turned up a price of $822 for the WO 22X70s; apparently the Tak22X60s are no longer available new, but the last price they sold for new was approximately $1150....I saw one used pair listed for 800-something GBP. :o

The Garrett Signature 22X85s cost $500.

After reading Edz's earlier posts on the Garretts, and especially his analysis of illumination issues, I bought a pair of the 22X85s....and immediately lost interest in using my "entry level" 25X100s and 22X100s....the premium Garretts are that much better.

Now, it seems that the WO and Tak 22's might well provide similar performance and illumination, in a smaller package.

Their prices, of course, are anything but smaller. Ahhh well, as always, you get what you pay for. IF you're lucky....

Cheers,

Jim

#17 RichD

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Posted 22 April 2009 - 06:05 AM

Sorry, missed that (trying to read quickly at work)

#18 Mike Hosea

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Posted 22 April 2009 - 01:47 PM

The reason I was poking (half joking) is that the exit pupil is the superposition of an infinite number of exit pupils, each corresponding to a different point in the view. Every complete visual telescope system has an effective aperture geometry for any given point in the view. Hopefully, this is circular and matches the quoted objective diameters for points near enough to the center of the view (but you have often observed otherwise). Since the binoculars in question here all have this property, and probably they have similar transmission percentages, the 70mm binos should be brighter paraxially. That is to say, you should be able to see fainter stars dead center in the view. However, the area of the effective aperture gets reduced sufficiently far off-axis because something obstructs part of the light cone from the objective lens. As you say, it's almost always a prism, but eventually the field stop of the eyepiece will completely obstruct some light cones. It's the curse of being a mathematician to see these somewhat different physical things as different examples of the same abstraction. Anyway, because of their superior illumination profile, the Taks should see deeper sufficiently far away from the center of the field. However, pressing further afield, as soon as we run out of apparent field in the Taks, the 22x70s should see a lot deeper (because the Taks don't show anything at all behind the field stop).

For a single datum to compare, the total illumination is OK, but a lot of complexity can hide behind it. Ideally, you could compare illumination profiles in graphical form, e.g. something like the attached. [snip] You could compare just over the apparent field angles they have in common, or you could add it all up for both and compare. [snip] I just pulled this data out of the air. It is normalized to the 60mm aperture (y=1 is 100% illumination of 60mm). [snip]

Attached Files



#19 Mike Hosea

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Posted 22 April 2009 - 04:35 PM

Ah, good, I screwed something up and nobody called me on it yet. The total illumination isn't the area underneath one of those curves, it's the volume under the surface of revolution. It turns out for the profiles above, the smaller aperture has 8.9% more total light over 46 degrees of AFoV. Extending to 66 degrees, however, the larger aperture wins out with 17% more.

I guess the picture isn't that much help to estimate things at a glance after all.

#20 EdZ

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Posted 22 April 2009 - 05:00 PM

I was waiting for you to pick that up Mike. ;)
edz

#21 harbinjer

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Posted 22 April 2009 - 05:06 PM

Very interesting Mike. I too saw the idea of the Riemann sum, but alas forgot the name. My math teachers should be proud. Also very good intuition on EdZ's part.

So it is possible to figure out what the actual size of the "hat" is, and how much deeper you can see? Also I'm not sure how AFOV figures into this. If you can see are larger field, then you must be getting more light, right? Does that mean the in narrower FOV the field stop blocking that light, or is it just being scattered?

#22 Dirk Jan

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Posted 22 April 2009 - 05:10 PM

I guess the picture isn't that much help to estimate things at a glance after all.

Ohw, in my opinion it certainly is. Visualizing it like that explained the issue at hand very much to me!

#23 EdZ

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Posted 22 April 2009 - 05:16 PM

There may be a difference in illumination across the fov and illumination across the exit pupil. Several years ago Anton Jopko wrote a paper as a follow-up to the method I proposed here for measuring aperture and illumination. There is a link in the Best OF Threads under measuring deficiencies. You can search the internet for it under Anton Jopko - Vignette. He speaks of a distinct diifference between the two, and what I believe I am measuring is illum across the exit pupil. I'm sampling with a 2mm dia light source on the objective, and the exit pupil is fixed by the power, regardless the fov, which is fixed by the diameter of the field stop. But I would encourage Mike and any others to read Anton's paper to help answer if I am correct in my thinking.

edz

#24 Mike Hosea

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Posted 22 April 2009 - 05:54 PM

So it is possible to figure out what the actual size of the "hat" is, and how much deeper you can see? Also I'm not sure how AFOV figures into this.


As Ed goes on to mention, there is a difference between illumination across the exit pupil and illumination across the view, if I understand what the former means. I probably won't have a chance to read the article that Ed points to, as I am sitting here at work and three things with a deadline of Monday just landed on my desk. I'll have to think more about what is being measured here and how useful it is. I guess my view is that brightness per unit area of the image projected on the retina is what really matters, provided there's not a lot of scattering from inferior optics, and provided the optics can actually deliver the light without too much aberration (a faint star is easily made invisible if not tightly focused). AFoV is not particularly relevant, since this (brightness per unit area) is a local rather than global thing. Brightness per unit area should be highest in the center of the view and lowest at the edge, generally. Most people (including myself) have difficulty seeing anything but a severe brightness gradient, but even if you can't see the brightness reduced near the edges, you may notice that the faintest things that show up in the center don't show up near the edge. The advantage of a better illumination profile is to show just as much, or nearly, away from the center as in the center. That's a nice feature for large objects and open clusters, cruising the Milky Way, etc.

#25 daniel_h

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Posted 22 April 2009 - 06:35 PM

obviously the Tak is the pick of the bunch (but the chances of getting one are buckleys) - which would people have out of the other two?






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