Liam Lawless (lawless@vignette.freeserve.co.uk)
Sun, 07 Mar 1999 15:44:21 +0000
Hi all,
It looks like some of you still misunderstand what I have proposed, so let
me try explaining again. Whether or not the idea works has nothing to do
with varying the inherent contrast of the emulsion by filtration (which
conventional wisdom says CANNOT be done with fixed-contrast materials, and
with which I would tend to agree), but with the fact that a lith emulsion
would be affected more by blue light than magenta light (magenta being
further up the spectrum, towards the red end). As Kevin has stated, the
"natural" colour sensitivity of the silver halides (to blue/violet/UV) is
extended into the green region in ortho emulsions, a point that I hadn't
considered and which means that the spectral response of APH is not as
narrow as I had assumed; if it doesn't work, I am pretty sure it will be
because the sensitivity range is not sufficiently narrow, and not for the
reasons that most of you seem to think.
OK, so we have a B&W negative in the enlarger with a magenta light source
above it (or should I say a magenta-filtered white light so no-one has to
correct me?) Ignoring the cyan filter for the moment, what reaches the
baseboard would be strong magenta light in areas corresponding to the
shadows of the negative, and weak magenta light in the highlight areas.
Now interpose a FAIRLY PALE cyan filter between lens and baseboard. My
point is that a pale cyan filter would have negligible effect on the colour
of (strong) light passing through the shadows of the negative - which is
thus still magenta when it reaches the baseboard - but a much larger effect
on the colour of the weaker light that has passed through the highlights of
the negative, which would become more or less blue. This colour change is
brought about by the cyan filter subtracting or holding back a good
proportion of the red component of the (weak) magenta light but passing the
blue fairly freely. And in this way I think it may be possible to print the
shadows by magenta light (of relatively low actinic power), and the
highlights by blue, the effect being to lower contrast.
I'm not insisting at this stage that the idea will work exactly as
described, but I do not believe failure would be for the reasons that have
been given so far. Of course my thinking muscle isn't infallible and I may
be totally wrong for reasons that have escaped me, but, as I said at the
beginning, I don't think everyone has fully grasped what I intended.
A practical application of a vaguely similar principle occurs to me. Years
ago, Cokin used to market pairs of complementary-coloured filters (do you
get Cokin filters out there?), one for use over the camera lens and the
other over the flash. The idea is that with, say, a blue filter over the
lens and a yellow over the flash, the background (where the flash is
ineffective) is rendered blue, but the colour is cancelled out by the yellow
flash on the main subject and therefore comes out neutral (on colour film,
of course!)
Liam
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