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GO Gemini 20x80 and Oberwerk 25x100 ?Aperture?

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#1 EdZ

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Posted 15 March 2008 - 03:11 PM

Here's a simple test that shows the true effective aperture of these binoculars.

Both the Garrett Optical Gemini 20x80 WP and the Oberwerk Giant 25x100 IF are built on the same body design. It’s the exact same as the Oberwerk Mariner line of binoculars. Recent test showed me that every small Mariner was substantially obstructed such that the effective apertures were all about 15-20% less than the full objective aperture. This made me curious enough to revisit these two large models and retest both of them.

I now know that my older methods for testing aperture were substantially incomplete. Basically, I would measure exit pupil and if it checked out, I would declare aperture just fine. Recent testing of 30 small binoculars proved to me that that is not a sufficient test. So I began to employ three different tests for checking aperture.

For one of those tests it is necessary to first test magnification. That can be done, but with these two binoculars it becomes necessary to set up a tape measure about 150 feet away. Once magnification is known accurately, you can directly measure the exit pupil and multiply to get effective aperture.

The second test is a reading from a scale placed right across the objectives and read directly thru the eyepiece by using a loupe to magnify the exit pupil. This method may give a fairly accurate reading, but depending on the type of internal vignette, for instance if it is a baffle between the aperture and the prisms, the reading obtained may be significantly overstated. It will never be understated. So the caution with this method is, unless you take careful measures, you may get a result that is larger than the actual effective aperture.

The third is a direct laser projection thru the lens. This method is quite precise, probably the most accurate of the three methods. A target laser is placed on a glass over the objective. Sliding the target laser from one side to the other, you can measure the width of the objective that allows the very center of the target laser to exit. This is the effective aperture. By marking the edge metal housing of the target laser with a 3M sticky at both ends of the aperture limit projection points, it is easy to then measure the distance between the stickies.

Using the loupe method I got a reading for the GO Gemini 20x80 of 69mm clear aperture. I thought, this seemed really low so I tested using the laser. With the laser method I got a reading of 70mm.

Exit pupil in the GO Gemini 20x80 WP has been checked a dozen times and measures 3.8mm. Something didn’t seem right. How could this be?

I devised another method to check all the above measures. I have a lens cap that has been cut to a 70mm aperture. I used it for testng the GO Gemini when I compared it to the Ultra 15x70. I decided to put this cap back on the Gemini and recheck the exit pupil. Here’s what I got.

Exit pupil in the Gemini WITHOUT the mask is 3.8mm.
Exit pupil in the Gemini WITH the 70mm mask is 3.75mm.

Now, from this we can declare a few things;
First, a binocular masked to 70mm that shows a 3.75mm exit pupil is clearly NOT 20x.
If full exposed aperture exit pupil is 3.8 and masked 70mm aperture exit pupil is 3.75, then the mask is actually slightly smaller then the effective aperture.
If full aperture shows 3.8mm exit pupil and 70mm mask shows 3.75mm exit pupil, then, when full aperture is exposed, the Gemini 20x80 WP is not operating at 80mm.

OK this now seems to check out with my loupe test and my laser test. But something else needs to be different. If a 70mm mask produces an exit pupil of 3.75mm, then the actual magnification can't be 20x. But I know this. Placing a 70mm mask reduces exit pupil by only a very small fraction. Therefore true aperture is very slightly larger than 70mm. Therefore I now have an aperture test (70mm) at which I can determine real magnification from exit pupil. Magnifiaction would be 70 / 3.75 = 18.7x.

If I then use 18.7x times my full aperture exit pupil of 3.8, I get 71mm effective aperture, a value that agrees within 1mm with my laser test.

The GO Gemini 20x80 WP is actually 18.7x71, not 20x80.

I did exactly the same procedure with the Oberwerk 25x100 IF.
The laser test shows the aperture as 94mm.
Direct measure of the full aperture exit pupil gives a 4.1mm exit pupil.
The 70mm mask produces an exit pupil of 3.1mm
This masked exit pupil would indicate an actual magnification of 70mm / 3.1 = 22.6x.

If I then apply a magnification of 22.6x to the full aperture exit pupil, 22.6x 4.1 = 93mm. This checks within 1mm of the aperture I measured using the laser.

The Oberwerk Giant 25x100 IF is actually 22.6x93, not 25x100.


FWIW, I also retested an Anttler Optic 20x80.
The laser gives an effective aperture of 71mm.
Without the mask this 20x80 gives an exit pupil of 4.2mm.
With the 70mm mask it gives an exit pupil of 4.1mm
The 70mm masks gives an indication that magnification is 70/4.1=17.0x
Using this power times the full aperture exit pupil gives 17.0x4.2 = 71.4mm. That agrees within 1mm of the laser measured aperture.

The Anttler Optic 20x80 is actually 17.0x71, not 20x80mm.

Anyone can take a 100mm inside diameter lens cap, cut a precise hole to 70mm and check these measures. The 100mm cap fits tightly over both these 80mm objectives, and reversed, slides tightly into the front end of the 100mm objective. See for yourself.

edz

#2 Patrick

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Posted 15 March 2008 - 08:17 PM

Wow...very interesting. That sure makes every binocular out there suspect as to their actual size and magnification.

Patrick

#3 EdZ

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Posted 15 March 2008 - 08:25 PM

Well FWIW, I also checked both a Fujinon 16x70 and the Oberwerk Ultra 15x70. Both checked out to readings of 69mm and 70mm aperture. Based on the previous, my measures are good to within 1mm.

edz

#4 Beg

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Posted 15 March 2008 - 09:21 PM

Wow,

If I didn't think that you knew what you were doing with this "test", I would almost consider this "trashing a product". Which I know has been heavily frowned upon and heavily moderated on this forum.

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#5 charen

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Posted 15 March 2008 - 10:52 PM

As a consumer of these some of these products one feels slightly disappointed with the companies who sell them. They sell these products with the specifications as claimed by them. There are some reasonable large discrepancies.

1/ is this acceptable?
2/ do they know these actual specs. and not being truthful in their advertising?
3/ are they ignorant of the true specs?
4/ or just taking the original Chinese manufactures specifications as correct and advertising them as such without any checks / Q.A. involved?

Would also be interested in the actual specs. of some of the other popular binos sold to the public.
Where did the term ‘truth in advertising’ go to? I would like to see the companies respond.

Thanks EdZ.

#6 Robert A.

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Posted 16 March 2008 - 01:05 AM

I have been thinking of this ever since you came out about the 8x40 Mariner. I thought you would get to these measurements! Thanks for the honest work! In some ways I didn't want to know it, but Yes, I do want to know! May we enjoy them for what they are! If a company wants to grab more business, there are measurements that can help!
Thanks for your hobby of measuring binoculars!

#7 EdZ

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Posted 16 March 2008 - 06:52 AM

2/ do they know these actual specs.



Before this time, I doubt anyone but the manufacturer knew this. IMO, The most likely scenario is that both companies are being misled by the manufacturer. Based on what I know, I don't think there is anyone out there testing for this sort of thing. There are no QA tests for this sort of thing. Even I was mislead by my own data for 3 years. I wrote a major report on all three of these binoculars three years ago. I never picked up on this until recently. It was not until after months of studies and testing on small binoculars that I had a suspicion to go look for this.

edz

#8 DJB

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Posted 16 March 2008 - 10:24 AM

Hi Ed,

Good testing procs., to say the least. The results with the FUJI indicates that your method is indeed proper, in my opinion. Thank you. I am not only surprised but a bit miffed at the results. "Who woulda thunk it?"

Best regards,
Dave.

#9 eklf

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Posted 16 March 2008 - 11:09 AM

Perhaps these observations might further explain why the 15x70 Ultras equal and sometime excel the performance of some 20x80s (demonstrated from the data OF Edz's very detailed 15x70 ultra report) , since these have virtually identical aperture, and higher polish and coatings of Ultras compensate for the 2x-3x increase in magnification.

#10 EdZ

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Posted 16 March 2008 - 12:55 PM

What's wrong with this picture?

Nothing right, looks just like you would expect

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#11 EdZ

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Posted 16 March 2008 - 12:58 PM

until you turn it around and see that
one side is supposed to be 20x80
and the other side is masked to what should be 20x70

obviously the pupils in the previous photo shouldn't look like that.
specifics are all noted in the first post.

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#12 jrw11

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Posted 16 March 2008 - 01:03 PM

EDz, what have you found out about the Obie 20x80 standards. Are they the same as these?

#13 EdZ

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Posted 16 March 2008 - 03:08 PM

I no longer have the Standards so can't measure.

I did remeasure the GO Gemini 20x80. I created several more masks to check. I used a 70mm, 60mm and 50mm mask. Numbers were all over the place. So I checked my masks and found the actual mask diameters are 68.8, 61.5 and 50.8. So I remeasured everything. This time all the values came within 1% of the mean. For magnification I got 18.8x, 19.1x and 19.2x. For aperture I got 71.4mm, 71.8mm and 72.6mm.

So, it's fair to say the GO Gemini 20x80 is actually 19x72.

I did measure the Oberwerk 15x70. Using the 60mm and 50mm masks I got 14.9x and 15.2x The masks gave me aperture of 62mm and 63mm. Measuring aperture with the loupe, I got 63mm. Measuring with the laser I got 62mm.

So it's fair to say the Oberwerk 15x70 is 15x63mm.


I also rechecked the Oberwerk 25x100 and got 22.5x92.


The causes are several.

In the Oberwerk 15x70 it caused by a baffle near the back end of the objective tube. The baffle opening is too narrow. You can see when that baffle edge is lined up with the prism edge, it cuts significantly from the objective diameter.

In the Obie 25x100 and the GO 20x80, there are no baffles.

In the GO 20x80 the back end of the objective tube wall inside diameter is not open enough all the way around to allow all the objective light thru to the prism. On some sides it intrudes into the light cone and cuts off the aperture. But then where the light does pass the tube wall, then it hits the prism aperture. So we have tube wall interference and prism aperture that is too small for the location of the prisms in the light cone.

In the Obie 25x100 the inside diameter of the tube wall is sufficient to clear the light cone, but once the light cone reaches the prism aperture, the prism housing cuts into the light cone. So, here we have an objective to prism distance that is too close for the opening diameter of the prism aperture. Or if you prefer, here the light cones is cut off due to prism housing so it is a too small prism for the light cone. Take your pick. They are both effectively the same thing.

edz

#14 jrw11

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Posted 16 March 2008 - 04:48 PM

Thanks EDZ! We owe alot to you and your constant work!

#15 camvan

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Posted 17 March 2008 - 03:43 AM

hey Edz, being I don't have the equipment that you have to do these measurements, what would be a good way to test my 20x80 TWP's to see how they are regarding this particular issue?

#16 EdZ

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Posted 17 March 2008 - 04:54 AM

One simple way is look down the barrel. The very edge of the objective lens should line up with the very edge of the prism with nothing in the way. That's would show the most obvious situation. Of course that's not the whole story. But if that doesn't work, that's the first problem.

Other than that, there's at least two other conditions that could be problems that this simple test won't show. After that you need masks that you know the size of and a precise caliper and a good eye for reading exit pupils to less than 1/10th of a mm. Also you could tape a translucent scale to the window and use a loupe to observe the exit pupil, as outlined in the directions for checking aperture. If you want a more accurate measure to double check, you need a target laser. If you want to triple check you need to follow the procedure outlined months ago for how to check magnification.

edz

#17 EdZ

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Posted 17 March 2008 - 09:59 AM

As a consumer of these some of these products one feels slightly disappointed with the companies who sell them. They sell these products with the specifications as claimed by them. There are some reasonable large discrepancies.



Well I can understand that. That's fair enough. But, before you get riled up and direct all that disappointment down just one or two paths, just take a moment and think about this; The four binoculars in this thread that I have tested and reported on represent models that can be bought not only from Oberwerk, Garrett and Anttler, but also from, Orion, Celestron, Zhumell, Binocularsdotcom, Opticorp, Opticsplanet, Amazon, Adorama, OPT, Strathspey, Telescope Service, EBinoculars Barska, Burgess, to name a few. Also several of those websites have lengthy tutorials about binoculars. Not one of them has described to you how to determine this deficiency, nor has any one of them ever reported it in their notes to specifications. That leads me to believe, they just don't know. I doubt this is part of any 20 point inspection.

edz

#18 ebone

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Posted 17 March 2008 - 11:07 AM

This measures are really impressives.
I would like to add some questions to the topic, about "why" this is happening to some production lines:
1) Bad engennering?
2) Quality control failure?
3) Intentional diaphragm of the optics to hide some distortions?
4) Reduce price when using internal components below the needed specs?
5) Other?

#19 EdZ

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Posted 17 March 2008 - 11:38 AM

This measures are really impressives.
I would like to add some questions to the topic, about "why" this is happening to some production lines:
1) Bad engennering?
2) Quality control failure?
3) Intentional diaphragm of the optics to hide some distortions?
4) Reduce price when using internal components below the needed specs?
5) Other?


Some other possibilities, not to say those above are right or wrong. In fact I think some of the above may be good possibilites.

Engineering to a moderate price point for the masses

Using the same design stretched to fit numerous sizes, when the design it not optimized for any one of those sizes, potentially saving cost of engineering a new design, but obviously losing in the translation from one size to another.

These are not the only binoculars that are affected in such a way. In the Small Binocular study of 30+ binoculars, I identified at least 10 others that are reduced in aperture by anywhere from about 10% to 20%. I identified a number of these several months ago.

edz

#20 JAS

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Posted 17 March 2008 - 01:51 PM


I was about to purchase a pair of 25x100s or 28s. But this has stopped me for the moment.

For those of us who are beginning in this hobby, what are the rea$onable alternatives? Or are there none, really? I assume that the Garrett Signature series has the same aperture mismeasurement problems. Also-is there reason to assume that the higher end big binoculars and/or binoviewers are rated correctly? Of course, one solution is to simply live with the knowledge of what the specs actually are, bite the bullet and move on...
Thanks-
JS

#21 EdZ

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Posted 17 March 2008 - 02:11 PM

I assume that the Garrett Signature series has the same aperture mismeasurement problems.


As I stated above in this thread I found NO such problems in the Oberwerk Ultra 15x70. the Signature series is the same model as the Ultra.

I wrote plenty last year documenting binoviewer specs. See the resources links at the top of the binoviewer forum for the thread on clear aperture.

edz

#22 eklf

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Posted 17 March 2008 - 02:16 PM

I assume that the Garrett Signature series has the same aperture mismeasurement problems. Also-is there reason to assume that the higher end big binoculars and/or binoviewers are rated correctly? JS


Well FWIW, I also checked both a Fujinon 16x70 and the Oberwerk Ultra 15x70. Both checked out to readings of 69mm and 70mm aperture. Based on the previous, my measures are good to within 1mm.

edz


Js,

As noted by Edz, the higher end ultras, which are identical to the signature series, do not have this problem.

#23 Rich V.

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Posted 17 March 2008 - 02:36 PM

These "enlightening" details certainly make a quality 15/16 (and likely 18)x70mm a more attractive alternative to larger but not necessarily much more capable "80mm" binoculars.

At least the premium 70mm binoculars deliver what they claim! A 70mm binocular is still a relatively compact package that isn't all that demanding when it comes to a mount.

I thank EdZ for his usual thorough evaluation as well as his open mindedness to keep rethinking his data and methods. :bow:

Rich V

#24 ronharper

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Posted 17 March 2008 - 02:43 PM

Stellarvue used to market a bino of this series that other dealers call a 15x70. They came up with a way to be both honest and put a positive spin on things. They (and Orion too, for a while) advertised it as 15x63, but with "oversized objectives". They claimed that masking the objective improved the optical quality, by getting rid of the most aberration-laden part.

Theres's something to that philosophy. Recent experiments with my Fujinon 16x70, both stopping down, and center-blocking the objective lens to use only the outer part, show that the imperfection in the images of bright stars does indeed come by and large from the outer 5mm or so. Not that the Fujinon is what you'd call bad at full 70mm. And also not, in my experience, that this series Chinese are so great even factory masked. But, it's gotta help with sharpness. I think it likely that if you could see a "20x80" that was really 80, it would have worse problems than a misleading spec.
Ron

#25 EdZ

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Posted 17 March 2008 - 02:47 PM

Orion too, for a while) advertised it as 15x63,



Orion did carry a 15x63. it was actually a 15x63, not a masked 15x70. I owned one for a while.






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