Re: Help Wanted


Charles Steinmetz (csteinmetz@redneck.efga.org)
Sun, 07 Mar 1999 07:05:28 +0000


Unfortunately, I don't believe the technique will have the desired effect,
for two reasons. First, it requires a printing medium that changes
contrast according to the color of the exposing light. The most prominent
examples of such materials are the variable-contrast enlarging papers,
which incorporate high-contrast halide grains sensitive only to blue light
and low-contrast grains sensitive to blue and green light. The
iron-sensitive proceses also exhibit some contrast color sensitivity.
However, silver halide emulsions show very little contrast color sensitiity
unless contrast-control is specifically designed in.

Second, the sort of control you are seeking would seem to require colored
negatives. Here's a thought experiment: imagine one filter between the
lamp and the negative carrier, and one between the negative and the
material being exposed. The color of the exposing light is determined by
the filters (and, of course, the spectral output of the lamp). Now, no
matter where in the system one introduces neutral density, it reduces the
intensity of the exposure but does not change the color of the light.
There is nothing magical about putting the neutral density between the
filters. What you would need is a negative that changes color as it
changes density, which would change both the amount and the color of the
light reaching the easel. This, by the way, is precisely the claim made for
pyro negatives and their special printing qualities on variable-contrast
paper.

Best regards,

Charles



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