Quote Originally Posted by Ed Sukach
Quote Originally Posted by Lemastre
Project the pinhole as large as you can get it with your slide projector or enlarger and ratio the diameter of the projected image with something of known length projected to the same enlargement. This gives the actual pinhole diameter. The diameter of the pinhole is not all that critical as long as it's very small and reasonably clean-edged.
I don't think this would work - the theory behind the "pinhole" is to diffract the light passing through to the extent that it would give a "lens- like" effect... bending the rays to a "pseudo" focus plane. I've tried measuring small spaces on a Optical Comparator, and the edges become *extremely* fuzzy and indistinct. If they did not, it is likely that the "pinhole" is too large for use.
I fear my suggestion was not clearly stated. It was just a way to measure the diameter of the pinhole by blowing it up large enough to measure with a millimeter scale so that measurement could be compared with another item of known size blown up the same amount. Once you know the actual diameter of your pinhole, you can figure its f stop value. A pinhole's diffraction properties don't affect its ability to be projected.