Help Wanted


Liam Lawless (lawless@vignette.freeserve.co.uk)
Sat, 06 Mar 1999 20:42:13 +0000


Hi everyone,

This concerns an idea that popped into my head when I couldn't sleep last
night (actually, 6.55 AM this morning) - it sounds crazy, I know, but I
can't see why it shouldn't work. How well I don't know, but I'm not able
to test it myself as I don't have a colour enlarger or colour printing
filters, so what I'm hoping for is someone to tell me where my logic is
wrong, or for someone who is convinced by it to give it a try and see if it
works.

Going back to Joao's original query about contrast control for lith
reversal, what I think is needed, were such a thing possible, is a short
base exposure for the shadows and a long base exposure for the highlights.
I believe it may be possible to do something similar with colour filters.

                  WHEN YOU'VE ALL STOPPED LAUGHING, I'LL CONTINUE....

I'll explain carefully, and apologise for the fact that this means talking
down to most of you. Now, a lith emulsion, as I understand these things, is
not colour-sensitised, which means that its sensitivity lies predominantly
in the blue-violet (and UV) spectral region. Correct so far? If so, then
light of different colours will have different actinic power as far as lith
is concerned, depending how much or how little blue its colour contains: a
blue light affects the film more than a green one (blue+yellow), and a green
more than orange (yellow+red), and so on.

The important thing is that I would expect a blue light to affect lith more
than a magenta light (blue+red) of the same intensity. Now, suppose we
project an original camera neg onto lith from a colour enlarger head with
maximum magenta filtration dialled in, and a pale cyan filter (say about
25C) held under the enlarger lens. Lots of magenta light will pass through
the shadows of the negative and its colour will be relatively unaffected by
the pale cyan filter that it must also pass through, but in the highlights
where only a little magenta light can pass, the effect of the filter under
the lens is much greater, and magenta+cyan, according to the rules of colour
printing, equals blue, which has more actinic power on lith. The same
filter factor applies to both shadows and highlights, so what I believe we
have done with our filters is to hold back the shadows and augment the
highlight exposure at the same time.

Well, am I mad??

Liam



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