Langford's Basic Photography, 10th Edition

Appendix A: Optical Calculations

Pinholes

The best size pinhole for forming images has to be a compromise. It must be small enough to form quite tiny circles of confusion, so that as much subject detail as possible can be resolved. But the smaller the hole, the more diffraction increases, so that eventually detail no longer improves and rapidly becomes worse.

Optimum pinhole diameter =√distance from film/25

So for a pinhole placed 50 mm from the film, the best diameter is the square root of 50 divided by 25 = 0.3 mm. To make the pinhole, flatten a piece of thin metallic foil on a pad of paper. Pierce the foil gently with the tip of a dressmaker's pin. Check with a magnifier that the hole is a true circle and free of ragged edges. By placing ...