Quote Originally Posted by ilona
Thanks Ed! I didn't realize until I read your message that this 85mm film dimension was the diameter absolutely needed by the film. But of course, the diagonal is also the diameter...
You lost me here. The diagonal of a "6cm x 6cm" (actually a lot closer to 5cm x 5cm ... measured it) frame is about 70mm and that would be the necessary image circle. An 85mm focal length, with an image circle of 163mm is considerably more than necessary. OK to have more than you need, not good - at all - to have less.

However, my previous question was also about "what is a safe margin between the diameter absolutely needed by the film (or diagonal) and the diameter of the image produced by the hole (or coverage)": 1cm, 2cm, 4cm extra on each side???
I don't have an answer to that. It depends on how much "fall-out" (dimness at the extremes of the frame) you consider to be acceptable. Some will tolerate - or even WANT more than others.

I can only suggest that you try it - and see what happens - which is pretty much what is necessary in pinhole photography anyway.