But if your aim, Thom, is to have a circular image in the middle of your paper/film format, the bellow mentioned rule will not work for you, in such a case you should perhaps assume a 5 times the focal length or even more. Use also thick material to make your pinhole.
Guillermo PS: it was somewhat hard to answer this nature of question without recurring to use "derivative calculus" which BTW, I thought it was called trigonometry, taught in grade 8 to me. My friend Juan use to say: "why are we learning this stuff we would never use", he was probably right ;-) ----- Original Message ----- From: "Tom Miller" <twmil...@mr.net> > The rule of thumb is that, on a flat film plane, the circle will be > 3.5 times the focal length. So, if the focal length is 1" the circle > will be 3.5", if the focal length is 2" the circle will be 7", and so > on. The f-stop doesn't really matter for this rule of thumb. > > ----- Original Message ----- > From: "Thom Mitchell" <tjmi...@ix.netcom.com> > How can I quickly determine the size of the image circle for a given > focal length and f/stop. I want to be able to keep an image circle > from getting too big, i.e. I want it to be 2' or 3''. Any quick help > would be appreciated as would simple rules of thumb as opposed to some > of the derivative calculus I sometimes see. Thanks for your help in > advance. Thom
