Telescope Reviews: GO Gemini 20x80 and Oberwerk 25x100 ?Aperture?

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

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