Tools of the Trade



(Published in the April, 1995 issue of Birding)

When I bought a Kowa 77 mm scope several years ago, I seriously considered getting the TSN-4 "fluorite lens" (i.e., low dispersion glass) model at nearly twice the price of the TSN-2, which is essentially the same scope with a conventional glass objective lens. The wisdom of the time held that the TSN-4 was "only 15 percent better [than the TSN-2] 10 percent of the time. Personal comment, Fred Beeler, proprietor of Christopher's Ltd., 11/13/92.

Close " Its somewhat better color fidelity was noticeable only in low-light conditions. I decided 10 or 15 percent more quality was not worth nearly 100 percent more money and bought the cheaper TSN-2.

My perspective changed when I compared a Bausch & Lomb (B & L) Elite 77 mm "ED Prime" scope to both Kowa models in the field and did some technical research on low-dispersion glass. My conclusion: Low-dispersion glass (represented by ED Prime in B & L's 77 mm and fluorite in Kowa's TSN-3 and TSN-4 models) provides much better resolution (sharpness of definition) at very high magnification. It also yields better color contrast, which is most apparent in the lowest light. However, at lower magnification and in ordinary light conditions, its superiority is barely noticeable.

Translated into a purchasing decision, whether to spend more money on low-dispersion glass 77 mm scope should depend on what kind of power eyepiece the purchaser plans to use. The TSN-2's resolution and color fidelity both appear excellent at 20x or 30x. Low-dispersion glass would not make much difference at those powers, so why pay for it? But birders who use 20-60x zoomlenses will often appreciate a 77 mm with low-dispersion glass. Both the B & L Ed Prime and TSN-4 models hold their resolution almost to or at 60x unless tripod movement or heat distortion prevents it. No matter how distant the object, colors remain true so long as they stay visible. The TSN2's resolution starts to blur above 40x. It may also produce slight color shifting ("chromatic aberration") if the object is far enough away. I suspect that other conventional glass 77 mm models. E.g. Bausch & Lomb's non-ED Prime Elite 77 mm. Close perform similarly in the field.

The technical explanation for the superiority of low-dispersion glass is that it minimizes the prismatic effect of ordinary glass -- i.e., disperses less white light into its component colors. Scopes with low-dispersion glass objective lenses best bring all three primary colors into focus simultaneously, whereas at least one primary color always remains slightly out of focus using conventional glass. Personal comment. John Cross, PHD, Senior Project Engineer, Bausch & Lomb, Sports Optics Division,Overland Park, Kansas. See also, Advertisement for Kowa optics, page 7, par. 3, ABA Sales, Annotated Catalogand Pricelist, Fall 1993, Colorado Springs, Colorado.

Sharper focus means better resolution. When color dispersion is minimized, colors are also contrasted better, which makes the image appear even sharper. Chromatic aberration is avoided. At 30x, the differences between low-dispersion and ordinary glass scopes are only half as great as at 60x. At 20x they are only one third as great. Therefore, while resolution, color contrast and color fidelity differences are still present at the lower powers, then they are so close to the limits of the human eye's own resolving and color differentiation capacities as to be barely noticeable, if at all.

Since low-dispersion glass' superior resolution is a function of color focusing, it may not be demonstrable on a black and white collimator in the laboratory. But I proved to myself that it can make a big difference for identifying distant birds in the field. I invite the reader to use the field testing methods described below before buying a 77 mm scope. I believe my results can be replicated.

I tried to make my field testing methodical. First comparing the B & L to the TSN-2, then to the TSN-4, I eliminated many variables by mounting quick release plates on each scope and alternately viewing the same object through each from the same tripod. I made a comparison sheet and noted the time, distance, light direction, light intensity and object description for each comparison. I tried to find or simulate situations where a critical field mark on a bird might be visible through one scope and not the other. Some comparisons involved birds. Others did not.

I tested for resolution by finding something I could read with one scope and not another at 60x. I in my case it was a 3/8-inch-high number on a parking sticker on the rear bumper of a car I measured to be 260 paces from my tripod. I could read all of the digits with the B & L ED Prime and TSN-4 scopes, but not with the TSN-2.

I tested for color fidelity several ways. On a lake on which gulls swam at various distances, I looked at their eyes, eventually finding a Herring Gull whose yellow iris I could just make out with the B & L ED Prime scope but which looked dark with the TSN-2. I also looked at the heads of ducks and the legs of gulls at distances and in lighting conditions where it was difficult to see colors well. At more than 40x or when the subject was backlit or there was glare or haziness, there were times when I could make out the head color of a Redhead or Common Merganser with the B & L Ed Prime scope and not with the TSN. When I made similar comparisons between the B & L and TSN-4 scopes, they performed about equally well.

One experience with a group of five birders demonstrated dramatically how very slight differences in scope quality can sometimes be critical to a correct bird identification. We were scoping a flock of Ring-billed Gulls standing too far away to see leg color clearly without effort. I spotted a larger gull among them. In the two B & L Ed Prime scopes we had among us, its legs appeared pinkish-gray. In two TSN-2s, they were gray with a slight tinge, which varied from pinkish to yellowish. In a Bushnell Spacemaster equipped with a 45x zoom eyepiece, they appeared greenish. The bird was too distant for us to be confident about iris color or mantle shade. It was a Herring Gull. But a careless birder, using either the Spacemaster or TSN-2, might have misidentified it as a California Gull, quite rare where I was in Eastern Kansas!

Incidentally, I sold my Kowa TSN-2 and bought a B & L 77 mm ED Prime. I believe that the more expensive scope effectively extends by 50% the magnification at which I can see sharp images -- i.e., from 40x to 60x. But, if I were not a zoomlens user who greatly values resolution up to the highest magnification, I probably would have kept my TSN-2.

While I am thoroughly convinced that low-dispersion glass should make a 77 mm scope worth approximately $500.00 more to the 20-60x zoomlens user, I am not offering my findings as the last word on scopes in the $1,000.00 class, which includes the Optilyth 80 mm, Swarovski 80 mm and Nikon 78 mm Fieldscope ED. The Nikon 78 mm Fieldscope ED uses low-dispersion glass. The Swarovski 80 mm and Optilyth 80 mm do not, but their larger objective lenses theoretically could increase resolution by 3.75% and admit 7.5% more light than 77 mm lenses, factors which may balance the benefits of low-dispersion glass to 77 mm models. I have not yet seen the Optilyth or Nikon models, but I am very impressed with the Swarovski. When I compared both the Swarovski ST80 and AT80 models to my B & L, I found their resolution just as good at 60x. The Swarovski's color contrast seemed a bit pale compared to the B & L's, but I could not find a bird whose coloration could be seen with one scope and not the other. Unlike my B & L, which has fogged up twice on cold, rainy days. I believe I can solve the fogging problem by purging the B & L scope with nitrogen and leaving thezoomlens eyepiece in permanently, but I will have to give up thoughts of using the scope as a long lens formy camera, and it will require repurging every time it is cleaned.

The Swarovski 80 mm has a transparent seal where the eyepiece inserts that allegedly makes it "waterproof" and "dustproof" 5. See, Advertisement for Swarovski optik, page 13, par. 1, ABA Sales, Annotated Catalog and Pricelist, Fall1994, Colorado Springs, Colorado. The Optilyth 80 mm has a similar seal.

The Optilyth and newest model Kowa scopes have similar seals, which at least keep out dust. If the seal really works, it is a must for anyone who plans to change lenses or use a camera adapter frequently. These and other differences should be the subject of more careful comparisons, and perhaps more discussion in Birding, before anyone makes a final judgement as to which $1,000.00 class scope is the best on the market.

Bob Fisher