Light fall-off furmula

Hi friends,

I'm looking for a formula to calculate the light fall-off from center to
edges.

I'll be working on a giant pinhole this week-end (8 feet x 9 feet) and I
need to anticipate the amount of dodging required at the center of the print
in order to get a fairly even exposure across the paper negative.

As a rule of thumb, I've been working on the basis of Eric Renner's book -
where he states that the maximum diameter is equal to 3.5 times the focal
length. However, I note that Larry Fratkin's calculator works on the basis
of just under twice the focal length.

What I'm looking for is a formula that will calculculate the amount of
fall-off from the center of the image. Alternatively, what would be the
diameter of the circle with minus-1 stop exposure, minus-2 stop exposure,
minus-3 stop exposure times.

Guillermo, how does one calculate the light fall-off with a pinhole sieve?

Failing all this, I guess I'll have to work with a light meter. But, I'm
not really satisfied with this option since the differences in reading could
result from either the scene or the light-off.

Thanks for your comments,

Guy
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Received on Mon Jul 25 09:14:18 2005