Re: Light fall-off furmula

Am Montag, 25. Juli 2005 18:14 schrieb Guy Glorieux:
> 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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Hi Guy,
I think to remember that the fall-off is the cosine to power of 4. In the
center you have the angle 0 and the cos(0) is 1. perpendicular to the axes of
the hole the cos(90) is 0. The cos(45) is 0.707 and to the power of 4 the
result 0.25. At an angle of 30 degree, the fall-off is 0.562. But please
verify my guess!

An idea to mesure the fall-off with a light meter you may try to "photograph"
a strong light situated in the pinhole axis. That eliminates possible
influence by the scenery.

May be it helps.

Best regards

Bernhard
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Received on Mon Jul 25 10:20:18 2005