PINHOLE DIAMETERS
The operation of a pinhole camera is often explained using a diagram like the left-hand one below, with light rays passing through a perfect pinhole. However, an ideal pinhole has zero diameter which would let no light through, so what diameter should a practical pinhole be?
A larger diameter pinhole admits more light, which is good, but as the middle diagram shows, this allows several light rays to pass from points in the object to the sensor, which will reduce sharpness. But if the pinhole is too small, light rays spread out due to edge diffraction (righthand image) and, again, sharpness suffers. Clearly there’s an optimal size. Equations differ, but try the calculator at https://tomroelandts.com/tools/ pinhole-size-calculator with the default 550nm wavelength. For a focal length of 16mm, as in our RPi HQ Camera pinhole device, this works out at 0.178mm, so we used 0.2mm. We have to admit that making your own accurately sized pinhole is difficult, which is why we recommended going to pinholesolutions.co.uk.
The aperture (f-number) is an important characteristic of a lens. This is the diameter of the lens as a fraction of its focal length and values of between f/2 and f/22 are common, the larger the number representing a smaller aperture. As well as affecting the exposure, the aperture affects the depth of field, with a smaller aperture giving a larger depth of field. The HQ Camera’s 16mm focal length, 0.2mm diameter pinhole has an unusually small f/80 aperture, which explains why dust specs on the sensor are focussed enough to be visible.