Vic Smith:
>Also, I have read that scopes such as the Kowa TSN-3/4 with flourite-coated
>lenses provide no measurable performance (brightness, color) improvement
>over similar scopes (ie TSN-1/2) without the flourite coating when viewing
>at magnifications under 40X. Conversely, it seems that if you want good
>performance at higher magnifications you need the flourite coated lens.

First, unless they're using a totally different technology than the
makers of telephoto lenses, it's not a fluorite coating (again, a
flour-rite coating would be opaque white), but rather an element
ground from artificially-grown fluorite crystal.

When light's refracted, different colors "bend" at different angles
(thus the prism and rainbow).

When a lens is made of glass elements that each have the same refractive
index, it is in essence very difficult to get three colors to focus
at the same point due to this prismatic effect - correcting for this
introduces different errors which need to be corrected which cause
more problems etc etc etc.

Calcium fluorite refracts light in a way complementary to regular
optical glass. The designer can use this fact to combine a convex
fluorite element with a concave optical glass element and, relatively
simply, cause the three primary colors to focus very close to the
same point.

There are also certain kinds of optical glass, made with rare earth
elements added, which also exhibit this kind of dispersion. It's
called "Ultra-low Dispersion (UD)" glass by Canon, "Extra-low Dispersion
(ED)" glass by Nikon, etc. While not as perfectly complementary in
its refraction index as calcium fluorite, when used properly lenses
made with combined normal optical glass and "UD" glass perform
nearly as well as those which use fluorite elements.

Now, this prismatic effect is not noticable at small magnifications,
which is why camera lens manufacturers don't generally use "UD"
glass or calcium fluorite in lenses shorter than about 200mm (four
times "normal"). The effect is there in short lenses, but overwhelmed
by other effects and indeed perhaps by diffraction if the lens is
short enough.

In the case of spotting scopes, where does improvement become noticable?
I don't know if 40x is the right answer or not. Photographers are
fanatics about measuring performance of lenses objectively, but
all I've seen for scopes are subjective measurements. This is OK,
because you're just going to look through the damned thing, after
all, and your subjective opinion is all that counts! Photographs
submitted for publication are scrutinized far more closely, by
professionals with very high standards for sharpness.

Also, quality of the objective is also going to be of paramount importance.
They also are going to introduce abberations, and zoom objectives will be
worse. On the one hand, the better the image formed by the scope,
the better the image output by the objective, but on the other hand,
if the objective is lousy it's going to mess up that pretty, well-corrected
image noticably! You might, for instance, be better off with the non-ED
or fluorite scope with a good non-zoom objective, than the
ED/fluorite scope with a good zoom objective.

My advice: try 'em. When working at high magnification, you're ability
to see things clearly are also going to be affected by stability - using
a high-quality scope on the ding-aling tripods most birders use is just
going to cause degradation of the image even in slightly windy conditions.
And those mudflats with those shorebirds often have wind. If you're
not willing to lug a decent tripod, say a good-sized Bogen, then I
doubt if you'll get the benefits of the extra-sharp image often enough
to justify the cost. You might be better off buying the cheaper
scope and using the savings to bribe your friends into carrying a
decent tripod for you.



- Don Baccus, Portland OR <donb at rational.com>