True up to the point where a significant percentage of the light is diffracted by the pinhole edge and your system becomes diffraction limited and then starts to deteriorate with decreasing aperture size. Eric Renner (www.pinholeresource.com & author of Pinhole Photography ISBN 0-240-803507) has come across over 50 ways of calculating optimal pinhole size in his research. His own conclusion is that optimal size is near:
Originally Posted by John Bartley
aperture diameter in thousandths of inches = sqrt(55*focal length in inches)
The book is a very worthwhile purchase, and covers slit apertures, zone plates, and other related topics. There is a sequence of eight 4x5 shots at apertures from f:22 to f:288 that is instructive. There are also plans for a simple wooden camera that takes 4x5 filmholders, similar to the Leonardo boxes I pointed out earlier as examples of simple, functional construction that you could easily duplicate.
Very nice job on the camera John. Your design is similar to the Leonardo, which uses eccentrically mounted dowels to provide a cam-like action to hold the film holder in place. Other options are bars with rubber band or spring tension to grip the film holder. A bit of self adhesive felt or soft rubber sheet is a nice addition where the face of the film holder presses against the rabbet/rebate (depending on whose English you're speaking).
I'll check Renner and post if he has anything useful to say about angles of view. My recollection is that this is highly dependent on the pinhole, especially the thickness of the pinhole material, and can approach 180 degrees if you have a good pinhole and either don't mind the falloff or curve the film/paper.