Despite the fact that I've been out using my last home-made camera, and the fact that I've got to build another camera for my rapid rectilinear lens that is sitting in the cupboard, I've got hold of a single lens from a projector (one of those projectors that used to be used in the classroom). I reckon the focal length to be about 320mm, but in order to get the depth of field I'm going to need to build (another camera, but also...) a diaphragm. But how far away from the lens does the diaphragm need to sit? Is it just experimentation to find the best spot, or is there a formula one should follow? Note, I'm not looking to work how large the aperture is, just where the diaphragm as a whole (hole?!) should sit. Thank you, David.