Sil Horwitz (silh@iag.net)
Sat, 06 Mar 1999 16:43:18 -0500
At 99/03/06 08:42 PM +0000, you wrote:
>
>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??
Well, not exactly mad, just lacking in differentiating between RGB and CMY.
You're thinking in RGB, but those darn filters are CMY, which means that when
you filter out everything but magenta (which is what the magenta filter does),
there is no effect on the cyan filter. We're talking subtractive here and
there's nothing to subtract except density. To make your system work, I should
think, you would need to use a white light exposure for the blue, and then cut
down on that with yellow. If the emulsion is absolutely, positively, sensitive
only to blue, all that will happen when you insert the yellow filter is to
remove blue (in proportion to the density of the yellow filter), and you could
get the same effect by stopping down. Now, if you have a source of separate R,
G, and B light beams, then your system would work as you theorized. But not
with filtered white light.
Incidently, I once taught color theory, and the hardest part was getting my
people to recognize that you have to think differently when it's RGB and when
it's CMY! There was always amazement when I used three projectors each with its
own R, G, or B filter, and the screen showed white when they were projected
together. Using filters in front of one white beam will always act as CMY.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net
Visit http://www.psa-photo.org/
Personal page: http://www.iag.net/~silh/
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