To simplify the formula for panchromatic film, multiply the square root of the focal length by .007 (all in inches) or use PinholeDesigner with a user constant of about 1.5. Reduce the diameter a bit for macro photography, increase a bit for ultra wide angle. Way Beyond Monochrome, 2nd ed. is a magnificant book, although the section on pinhole photography doesn't thoroughly address the often overlooked problems with wide angle pinholes. I prefer .002" brass shim stock over aluminum for making pinholes. Dimpling it before making the hole reduces thickness around the hole even more. It seems to work cleaner than aluminum. Brass can be chemically darkened. One small sheet of shimstock might last a lifetime.
Originally Posted by Nick Kanellos
Pinhole photography sounds simple, but like many simple arts, it demands more of the practicioner than something as complex as digital photography. Entire books have been written on the subject without covering all aspects of it: Eric Renner's fine 260 page Pinhole Photography, for example. The art and technique of pinhole photography are discussed at length on http://www.f295.org/Pinholeforum/