From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 11/07/01-06:44:51 PM Z
At 03:26 PM 11/07/2001 -0800, you wrote:
>Try using under-lens filters. They'll be out of focus, so
>shouldn't affect image quality too greatly if you treat
>them gently.
>
>Tox
>
The problem is that filters in the image forming path must be of very
high quality and in very good condition. The best filters are gelatin. They
are very thin and are homogeneous. Irregularities will cause variations of
focus across the image. Since a convergent/divergent beam of light is
affected by anything in its path the ideal filter is very thin. For
practical enlarging even a glass camera filter is probably thin enough to
avoid the theorectical problems with spherical aberration and chromatic
aberration, which are added to a vergent beam by a plane parallel plate: a
gelatin filter will be pretty much free of these both because of its
thinness and because its index of refraction is much closer to air than is
glass.
Blemishes on the filters definitely _will_ show up on the print so they
must be clean. Gelatin filters are quite delecate and not water proof.
Nonetheless they can have a long lifetime with carful handling.
I think Ilford makes thin filters out of a plastic material which is
waterproof. Its optical properties may not be quite as good as gelatin but
are close enough not to make a practical difference.
At one time Kodak supplied VC filters in 14"x14" sheets but I don't know
if they are still available. In any case using an under lens filter is
easier and the filters are readily available. I know Kodak makes filters
for this use, not sure of Agfa.
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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