Focal ratio is the so-called "F-stop" of your pinhole camera, which is calculated by dividing the distance from pinhole-to-film by the diameter of the pinhole. You want to use the same units for both, of course. It's most convenient to use millimeters. For instance, if my camera has a 120mm distance from pinhole to film, and the diameter of the hole is 0.3mm, then the camera's focal ratio is: 120/0.3 = F/400.
Originally Posted by aaronmichael
It's common to find the pinhole-to-film distance referred to as "focal length," although technically you can't focus a pinhole (or rather, pinholes are fixed focus). The distance is usually measured from the pinhole to the center of the film plane.
This is also a good time to mention that there are formulae for optimal pinhole diameters. I believe it was Lord Rayleigh who figured out that for a given "focal length," there's an optimal diameter of pinhole aperture for a give wavelength of light. Any larger and the image gets softer because of simple geometry, while any smaller and the image begins to get softer due to diffraction of the light around the edge of the aperture. There are people who obsess over this to the point of missing the bigger picture, which is that it's pinhole, and it's supposed to be (somewhat) soft focus. A more important point is to ensure your pinhole is as round and smooth as possible, and "in the ballpark," size-wise.
Anyway, I look forward to your images. Keep us posted.