A: Optical calculations
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 (page 326) increases, so that eventually detail no longer improves and rapidly becomes worse.
So for a pinhole placed 50 mm from the film, best diameter is the square root of 50 divided by 25 = 0.3 mm. To make the pinhole, flatten a piece of thin metallic kitchen foil on a pad of paper. Pierce the foil gently with the tip of a dressmaker’s pin. Check with a magnifying ...