John Brownlow asks about pinhole diameter. I did some experiments with a set of pinholes I have. It seemed to me that the optimal pinhole as determined by the formula did produce sharper results than smaller or larger pinholes.
The formula that I use does not refer to the outer diffraction ring but rather the size of the central diffraction disk. The formula assumes that the optimal pinhole is the same size as the diffraction disk it produces. But remember that this is dependent on wavelength so you may get different results using panchromatic film than with orthochromatic or blue sensitive film or paper negatives. Anybody who has a set of laser drilled pinholes can do this experiment, just make a camera optimized for one of the middle sizes and take pictures with different pinholes. You can photograph a lens testing pattern and actually measure the difference. Another interesting experiment would be to make the same size pinhole in stock of different thickness and see how this affects image quality and covering power.
