Quote Originally Posted by SMBooth View Post
for any given pinhole diameter there is a focal length
Poorly stated again to the point of falsehood. A pinhole has no focal length, as it doesn't focus. It has a pinhole-to-film distance.

For any given pinhole diameter, there is a range of pinhole-to-film distances that gives a reasonably good balance of point spread and diffraction. Theory also often assumes a perfectly made pinhole in an infinitely thin material.

See bowzart's post for other considerations that need to be taken into account.