At 10:19 PM 11/4/99 +0000, John A. Lind wrote:
>
>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.
>
tiffen claims that they grind and polish thier filters for the same purpose.
>This reduces risk of flare, unwanted distortion, and as I understand it
>coplanarity is important for reducing risk of "fringing."
>
sounds good.
>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.
yup.
>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).
>
tiffen claims that the glue has the same index of refraction as glass,
hence it behaves as one solid piece.
>As before, the question is magnitude of effect and whether it is detectable
>in the end product.
>
well, i do know that filters add flare. you can try this experiment: take
your camera with no filter on the lens and point it towards a small light
source. frame it so that the source is in one corner of the frame. observe
any flare. while holding the camera in this position grab a filter and
screw it on the front of the lens. with a white light source and an
uncoated filter, you will see a white blob of flare from the filter. we
only have one coated filter, a n*k*n brand, and it added a little green
blob of flare.
Be seeing you.
Dirk Wright
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