The diameter of the pinhole is crucial and affects your image in several ways. Take a deep breath and read on. It will all make sense, I promise.
When making or using a pinhole for photography it is useful to know the properties your pinhole will exhibit. The diameter of the pinhole will effect diffraction, interference and coma which are all detrimental to image quality although that might be your desired effect. You might also want to consider if your pinhole/focal length combination will produce an image that is too wide or too narrow to be usable.
The f-number in pinhole photography is exactly the same as for lens based photography. You need to know the f-number in order to calculate what shutter speed/ISO combination to use for your exposures. Remember that apertures used in pinhole photography tend to be much smaller (numerically higher) and will result in longer exposures times and greater depth-of-field.
Take the Focal Length (the distance from the film plane to the pinhole) and divide it by the Diameter of the Pinhole – (the size of the pinhole). A Focal Length of 50mm and a Diameter of the Pinhole of 0.6mm will give a f-number of f-83.
Use the table below to calculate your f-number if you don’t want to muck about with a slide rule. Focal Lengths (in red) are down the left side and Diameter of the Pinhole (in blue) along the bottom.
Table of f-numbers
Now that we know out f-number, Focal Length and Diameter of Pinhole we need to consider if the image it produces will cover the area of our recording medium (film or digital sensor) and the inherent qualities of the image it produces.
The size of film or digital sensor has a bearing on which focal lengths are considered wide, normal or long.