How the Pinhole calculator works

Summary

Calculator that turns a pinhole camera's hole diameter and focal distance into an f-number you can use with any meter, then reminds you about reciprocity for the long exposures pinhole work demands.

How it works

A pinhole camera has no lens. The image is formed by a tiny hole in a thin metal disc. There is still an aperture (the diameter of the hole) and a focal length (the distance from the hole to the film), so it still has an f-number, just a very high one. This modal works that f-number out for you.

Pinhole diameter

The width of the hole in millimetres. Pre-drilled laser-cut pinholes are typically 0.2mm to 0.5mm. Hand-pricked holes vary. A digital caliper or a microscope ruler is the only reliable way to measure one.

Focal distance

The distance from the pinhole to the film, in millimetres. This is the focal length of your pinhole camera. A shoebox might be 100mm. A 4x5 pinhole back is around 50 to 80mm.

The f-number

Calculated as focal distance divided by pinhole diameter. A 0.3mm hole at 60mm focal distance gives f/200, which is roughly 8 stops slower than f/16. That is why pinhole exposures are usually measured in seconds or minutes.

Reciprocity matters

Most pinhole exposures are long enough to trigger film reciprocity failure. Make sure your film stock is set in the Equipment modal so the meter adds the reciprocity time on top of the geometric calculation.