Re: Preflashing film

[T3D Kirk W. Charles <kirk@xxxxxxxxxxxxxxxxxx>]

 Greg Erker <erker@xxxxxxxxxxxxxx> Asks:

> Suppose I want to pre (or post) flash my film to
> reduce contrast slightly and bring out the shadow
> details. How best to do it?
>
> How much preflash is desirable?  I recall the number
> of 5 or 6 stops below the exposure given to the film.

First let us understand how preflash (or as I was taught fogging) works and
what it does.  Negative Film's response to light is not linear but has a shape
like shown
        |      D____B
        |      /
        |     /
Density |    /
       A|___/C
        |
        |________________________
           Light amount

Point C is the threshold below which nothing shows on the NEGATIVE except the
film base (that is why the density is not zero)

Point B is the maximum density of the film.

So you can see that if AC is large the amount of light needed to record
anything at all on the film is large and the detail in the shadows will be
lost.

The idea in Fogging (flashing) the film is to put just enough light on the film
to push it over the threshold point.

Get the curves from the film company. Normal exposure is between Points C and
D.  Fogging amount can be approximated from the curve.

The amount of development affects the slope of line CD
more development > steeper curve = more contrast.

Now as far as color film in concerned each color layer of the film has a
slightly different response curve.  So any fogging (flashing) will give color
shifts.

Back to Black and White
Tri-X has interesting propreties when making very long time exposures (2min and
longer.)  As exposure times get very long the film is no longer reciprocal.
 That is the aperture/time relationship no longer holds.  So by using very long
exposure time combined with under development on can compress a wide range of
highlight and shadow onto the limited range of the film (that is to say very
low contrast). T-Max film is designed to reduce the reciprocity failure and
thus does not work as well in this application.

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.


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