Telescope Reviews: Comparing GO22x85, WO22x70, Tak22x60

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.

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