Re: Preflashing film

[T3D William Carter <wc@xxxxxxxxxxxx>]

Kirk W. Charles notes:

>Any of the methods Greg mentioned should work but one must note that the time
>from exposure to fogging may be important.  Also a slow time fog  should in
>theory affect the highlights less if the filn has some reciprocity failure.

The reciprocity law failure is due to migration of the electrons, knocked
off during exposure, back to the silver halide. A too slow or too brief
exposure and the freed electrons tend to migrate back. Hence the cold camera. 

I would think that any exposure in which reciprocity law failure would be
significant, would be so low in the toe that your base fog would cover it
pretty effectively.

The main thing about flashing film is that it effects shadows but has little
effect on the highlights. If you think of a progression of numbers, such
that each number is twice the value of the one before, you have a good
approximation of what you can expect from some light value being raised a
stop. Say 2, 4, 8, 16, 32, 64, 128 etc. Now, if you were to add one unit of
light, so that your progression reads 3, 5, 9, 17, 33, 65, 129 etc., you'll
see that this single unit of light has a profound effect where there has
been little exposure (shadows), and virtually no effect where there is a lot
of exposure (highlights).


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