On Monday I tested the new Kowa TE-11WZ zoom on an 883 scope. It is not much bigger than the previous zoom, weighs 380g or about 55g more than the old version. I measured the eye-relief as 16mm/25x, 15mm/30x, 14mm/40x, 15mm/50x-60x. This is measured from the plane of the eyecup twisted in, not the eyelens glass surface, so is pretty well in accordance with Kowas specs. Quite easy to view, unusually low lateral ca, although I only assessed this subjectively by looking at color fringing of the coarse test patterns in the target when moved towards image periphery. Edge sharpness is not quite as phenomenal as in the ATX series, but easily good enough to be satisfying. Panning felt natural to me. Old zoom and new zoom, both set to nominal 30x and held side-by-side backwards in front of a fresh new snow snowbank in lightly cloudy, almost sunny light did not reveal visibly perceivable transmission or color balance differences between the zooms, so for all practical purposes the transmission should be identical to the old zoom. On the scope, the color bias is the familiar slight yellow tone that would begin to look natural in prolonged use, but is noticeable after the neutral-bluish Swarovski ATX 95 I now use as a reference.
I spent quite some time on the yard of the shop doing resolution distance tests (setting the scope to a distance from the target where the lines in all orientations are just becoming visible) between the Kowa and my ATX 95. This was obviously not so much for testing the zoom than testing the scopes themselves. All tests were done without a booster, to show differences in real field use. I used Jan Meijerink's resolution target, which has an ISO-like pattern of lines in four orientations. I used the fifth-smallest pattern, but even that one has the lines visibly thicker than the spaces, so it will give lower resolution readings than a perfect target would. There was a thin layer of new snow on the ground, and variable thin cloud coverage with occasional brief sunlight. I did not use the sunlight spells for setting the distances. Very little heat haze, but some at times, likewise with wind. This is a sample of the Kowa 883 that, at 60x, had shown pretty much flawless diffraction patterns indoors, with superb symmetry and no discernible astigmatism, coma or prism defects. The distance from which the target could be resolved with the 883 was 18.85m. For the most direct comparison I could think of, I used the Swaro ATX 95 as an "88" with an aperture mask and its zoom set to 60x that was earlier set with a booster to show the same number of millimeters at the same distance as the Kowa at maximum zoom. This setup resolved the pattern at 19.05m. I kept the Kowa in place on its own tripod for the entire period, so I could and did go back and forth between the scopes a lot to check that light had not changed too much and "resolved" would be similar between both scopes. ATX at 95mm with 60x mag resolved at 20.45m, ATX "88" at 72x at 20,65m and ATX 95 @ 72x at 21.75.
Looking at these figures with a calculator, the stopped-down ATX and 883 both at 60x were about 1% apart. This difference would be very small even as a sample difference between two identical scopes of the same make and model.
Looking at all the figures, it looks like I may have been less patient and more strict in determining the distance for the full magnification unstopped ATX, since the figures I get with that are a bit off from what they should be. I measured the distances only after having decided where to stop moving the scope, and there was definitely a thought in my mind that I should try to be conservative in determining the full-power full aperture distance at. Had I measured about 22m, the figures would look more consistent. As it stands, increasing aperture from 88 to 95mm for the 60x magnification increases resolution exactly as theory predicts (8%), and a little less for the 72x magnification. Increasing magnification from 60x to 72x increases resolution by 8.4% with the 88mm aperture mask, and 6.3% with full aperture, average of the two would be 7.4%. And the ATX at its full 95mm aperture and maximum magnification of about 72x gives it a 14% edge over this excellent specimen of the Kowa.
What this means is that had Kowa opted for an otherwise identical eyepiece but with 30-72x range, it would have effectively cut to half the resolving power advantage it yields to the big Swaro. Of course, these results are with my eyes only, but less sharp eyes would benefit from the extra magnification even more.
It also means that with this eyepiece, the Kowa 883 is for all practical purposes the equal of the ATX 85. Any differences in resolution are more likely to be sample based than design based, but given equally excellent samples, the 883 should resolve 3.5% better. This sounds like nothing, of course, but translates to a distance difference of 35 meters at one kilometer, which is not exactly nothing. Also, the ATX will have slightly better brightness, and has a slightly more neutral color cast. The brightness advantage of the stopped-down 60x ATX over the Kowa was visible, but not that substantial in this rather good light.
Kimmo
I spent quite some time on the yard of the shop doing resolution distance tests (setting the scope to a distance from the target where the lines in all orientations are just becoming visible) between the Kowa and my ATX 95. This was obviously not so much for testing the zoom than testing the scopes themselves. All tests were done without a booster, to show differences in real field use. I used Jan Meijerink's resolution target, which has an ISO-like pattern of lines in four orientations. I used the fifth-smallest pattern, but even that one has the lines visibly thicker than the spaces, so it will give lower resolution readings than a perfect target would. There was a thin layer of new snow on the ground, and variable thin cloud coverage with occasional brief sunlight. I did not use the sunlight spells for setting the distances. Very little heat haze, but some at times, likewise with wind. This is a sample of the Kowa 883 that, at 60x, had shown pretty much flawless diffraction patterns indoors, with superb symmetry and no discernible astigmatism, coma or prism defects. The distance from which the target could be resolved with the 883 was 18.85m. For the most direct comparison I could think of, I used the Swaro ATX 95 as an "88" with an aperture mask and its zoom set to 60x that was earlier set with a booster to show the same number of millimeters at the same distance as the Kowa at maximum zoom. This setup resolved the pattern at 19.05m. I kept the Kowa in place on its own tripod for the entire period, so I could and did go back and forth between the scopes a lot to check that light had not changed too much and "resolved" would be similar between both scopes. ATX at 95mm with 60x mag resolved at 20.45m, ATX "88" at 72x at 20,65m and ATX 95 @ 72x at 21.75.
Looking at these figures with a calculator, the stopped-down ATX and 883 both at 60x were about 1% apart. This difference would be very small even as a sample difference between two identical scopes of the same make and model.
Looking at all the figures, it looks like I may have been less patient and more strict in determining the distance for the full magnification unstopped ATX, since the figures I get with that are a bit off from what they should be. I measured the distances only after having decided where to stop moving the scope, and there was definitely a thought in my mind that I should try to be conservative in determining the full-power full aperture distance at. Had I measured about 22m, the figures would look more consistent. As it stands, increasing aperture from 88 to 95mm for the 60x magnification increases resolution exactly as theory predicts (8%), and a little less for the 72x magnification. Increasing magnification from 60x to 72x increases resolution by 8.4% with the 88mm aperture mask, and 6.3% with full aperture, average of the two would be 7.4%. And the ATX at its full 95mm aperture and maximum magnification of about 72x gives it a 14% edge over this excellent specimen of the Kowa.
What this means is that had Kowa opted for an otherwise identical eyepiece but with 30-72x range, it would have effectively cut to half the resolving power advantage it yields to the big Swaro. Of course, these results are with my eyes only, but less sharp eyes would benefit from the extra magnification even more.
It also means that with this eyepiece, the Kowa 883 is for all practical purposes the equal of the ATX 85. Any differences in resolution are more likely to be sample based than design based, but given equally excellent samples, the 883 should resolve 3.5% better. This sounds like nothing, of course, but translates to a distance difference of 35 meters at one kilometer, which is not exactly nothing. Also, the ATX will have slightly better brightness, and has a slightly more neutral color cast. The brightness advantage of the stopped-down 60x ATX over the Kowa was visible, but not that substantial in this rather good light.
Kimmo
