Lon Williamson wrote: "Question: Is it the absolute diameter of the aperature, or some formula taking into account diameter _and_ distance from the film that causes diffraction limiting?"
My understanding is that diffraction gets much worse as the aperture gets further away from the film plane. It makes sense when you think about it. Diffraction is a bending of light. If the light is bent 1 degree off it's path for a given aperture (I picked that number out of the air - I have no idea what angle it actually bends) then the amount of diffraction would increase as the aperture moves away from the film plane. Diffraction can become an issue in macro photography when you add extension between the lens and the camera. The effective aperture becomes extremely small and diffraction increases. This has a more in depth discussion of it: http://www.scubageek.com/geek/articles/diffraction.pdf Tom Reese
