I never heard of anyone using a slide projector to find the optimal pinhole aperture. Somebody else out there might have heard about it.
There are many formulas that various pinhole photographers have used. Many have their own personal formula. I found a formula in a Kodak publication. It seems to give me the most optimal pinhole. And I have experimented with different diameters. Here is the formula I use. The Square root of the Focal Length times 0.007 equals diameter in inches Look in the book, Pinhole Photography, Rediscovering a Historic Technique by Eric Renner which is in its 2nd edition. This book will show you many formulas. I'm sure people in this list have their own formula. Eric Renner also publishes the Pinhole Journal, and is owner of Pinhole Resource, a non-profit organization. The address for the Pinhole Resource is: Pinhole Resource Star Route 15, Box 1355 San Lorenzo, NM 88041 USA Some pinhole photographers do not want the most optimal aperture. They prefer a softer or blurred image. Lonnie
