From: jack@dcs.glasgow.ac.uk (Jack Campin)
Newsgroups: rec.photo
Subject: dark-field intensification (was: Postprocessing help!)
Date: 1 Oct 92 14:59:27 GMT
Reply-To: jack@dcs.glasgow.ac.uk (Jack Campin)
Organization: COMANDOS Project, Glesga Yoonie

gt4609c@prism.gatech.EDU (Scott Coffeen) wrote:
> I recently took some pictures at a concert with VERY low light using a
> cheap automatic camera with 200 speed film. OK, I agree, the set-up stinks
> but I had no choice.  Not surprisingly, none of them came out, however you
> can see a few specks here and there on the negative. My question is, is
> there ANYWAY to take the negatives back to the lab and get them to come out?

If this was colour film (and I don't think there is a monochrome 200-speed
film) you've had it; neither you nor the lab can do anything.  With black
and white, this sounds beyond the capacities of chemical intensification,
too; there are an assortment of these techniques described in places like
the Focal Encyclopaedia of Photography - strong selenium toner is the
easiest, others use chromium or uranium compounds.

Another monochrome technique I've just read about in an out-of-print book
from the 60s but fortunately haven't had to use, which looks like having
more potential in a desperate case like this: dark-field copying.

You set up your negative like this:


		|       \ light  \     |
		|        \        \    | baffle
		|         \        \   |
		|                      |
		|                      |
		|                      |
	BLACK   |                      +                  -
     background |                      +                -| |
		|                      + negative     ((   |  camera
		|                      +                -| |
		|                      +                  -
		|                      |
		|                      |
		|                      |
		|         /       /    |
		|        /       /     | baffle
		|       / light /      |

You must make sure that no light can go in a straight line from the lights
through the negative to the camera.

Silver is better at scattering light than absorbing it, so the faintly
shadowy grey bits that represent the highlights in the negative will
photograph as bright, while the black background will be what shows through
in the clear bits representing the shadows.  You end up with another
negative of higher contrast.  The example shown in the book was much more
dramatic than anything I've seen attributed to chemical intensification.

But dye (as found in a processed colour film, which has no silver left
in it) won't scatter light to a significant extent, so this technique
is unusable for any kind of colour film, be it slide or negative, or for
Ilford XP2.

I suppose for Kodachrome there might be a similar way of photographing the
relief effect to recover a monochrome image (crossed polarizers?).  That's
getting *really* desperate.

-- 
--  Jack Campin    room G092, Computing Science Department, Glasgow University,
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