At 09:01 11/4/99 , Dirk Wright wrote:
>moot. Also, how significant is the fact that Tiffens are laminated, with
>the dye in the glue, vs. the solid glass of the others? Tiffen claims
>greater consistency for their process and says that no solid glass can
>possibly equal the accuracy of their filters.
The absolute flatness of B+W and Heliopan with Schott glass has already
been mentioned . . . I will add that the front and back surfaces are
absolutely coplanar as well.
This reduces risk of flare, unwanted distortion, and as I understand it
coplanarity is important for reducing risk of "fringing."
Don't forget that *any* time light passes from one substance into another
there is at least *some* reflection (causing transmission loss in addition
to what is absorbed in the media). If the ray path is at an angle there is
at least *some* refraction also. That is, the light changes direction!
When this occurs, different wavelengths (a.k.a. colors) bend at different
angles. [Sir Isaac Newton's prism splitting white light into colors.]
This is the reason most lens designs I've seen have at least one group of
two elements with two types of glass (different indices of refraction)
cemented together . . . it uses refraction between the two glass surfaces
to correct for this effect.
In a "sandwich" filter you have four surfaces versus two in solid glass.
Since the middle "stuff" (whatever it is) is different from the glass,
there are four changes in the index of refraction on the way through versus
two in solid glass (counting the air on both sides of the filter).
As before, the question is magnitude of effect and whether it is detectable
in the end product.
-- John
P.S. As much as I like Schott glass, there are times when I prefer shot
glass; especially after spending the day photographing my favorite distillery.
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