Ah, okay, I missed what you were looking for. Assuming the diffuser is evenly lit, the intensity of light from the pinhole falling on the film plane is determined by the same inverse square law used to calculate flash exposures -- so you simply need to make the pinhole far enough from the film that there's less than 1/3 stop difference in the light intensity from center to corner, and ensure the diffuser is evenly lit. And, since you don't need to worry about image sharpness, you can simply make the single central pinhole larger if you find you need more light (which you probably will unless you use a 60 W bulb for a light source).Originally Posted byjdef

Let's see, 1/3 stop is cube root of 2, about 1.26x light intensity, and the square root of that is approximately 1.12 -- so the pinhole distance must be such that the distance to the corner is no more than 12% greater than that to the center of the film. For 8x10 film, the film radius, center to corner, is square root of 41, 3.45 inches, so a right triangle with one leg that size musn't have more than 12% different between the other leg and the hypotenuse -- so 3.45^2 + X^2 = (1.12 * X)^2 and thus 11.9 = 0.26 * X^2 and X (the distance from film plane center to pinhole) must be at least 6.77 inches. I'd personally make it longer...