Fall-off with pinholes in thin material is equal to the cosine of the angle raised to the 4th power. This means a pinhole camera with a corner to corner coverage of 90 degrees will lose two stops of illumination in the corner, or a bit more if the pinhole edges aren't perfectly sharp.

Quote Originally Posted by DWThomas View Post
And it just hit me, in addition to the pinhole appearing elliptical (smaller effective aperture) off-axis, the distance from the pinhole to film is also increasing, shrinking the effective size even more. So those two effects compound things. I could probably figure out an equation, but since I retired I don't do trigonometry.