There are formulas which allow you to optimize focal length to pinhole
diameter for a given wavelength of light. There are a few web sites
where people have tables with these values - I can't remember the URL's
so you'll have to check my page for the links. When you use the and
optimal aperture/focal length combination you get something that looks
pretty much like an old soft-focus portrait lens or just an old lens with
uncoated optics.
Of course most people are not interested in doing pinhole to get a
picture that look almost exactly like a lens photo but takes ten times as
long to make. They want lots of blur, and huge distortion, and lots of
falloff on the edges. If this is what you are after then don't use a
commercial pinhole - you're best best is to ram a needle through a piece
of aluminum foil, stick this on a box with some gaffer's tape and GO!
richard u wheeler
rwheeler@everest.hunter.cuny.edu
rwheeler@guggenheim.org
http://everest.hunter.cuny.edu/~rwheeler