Re: Light fall-off furmula

Guy,

Here is a suggestion from an intuitive non math person.

Would it be possible to place a standard target out side the camera
while you measure the light? For example, if it is possible to hang a
large sheet of white foam core in front of the pinhole you might be
able to have a consistent white target subject image to meter on your
film plane.

This of course is ridiculous if you are using an upper room in a tall
building. I wish you the best weather for your exposure! And a large
developing trough and a coordinated friend for processing!

eric nelson
marshfield wisconsin usa

On Jul 25, 2005, at 11:14 AM, Guy Glorieux wrote:

> 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 Wed Jul 27 07:25:28 2005