I'm thinking about making a *really* wide-angle flat negative (not a curved neg) ph camera (haven't decided about what material yet).
My question is, how short can the focal length be for a certain negative size? When ph cameras have short enough focal length, the image start to get vingetted (fading to almost black at the edges (on the positive print)). Correct me if I'm wrong but the light fall-off is due to two different things. 1) The distance from the ph to the neg increases as you get farther from the center, and therefore you get less light intensity. 2) The round ph turns more and more into en ellipse (as viewed from the neg) as you get farther from the center, and therefore the effective f-stop increases. Both these factors work together to form an equation that determines how much light reaches any point on the negative and would assume that the parameters are: distance-from-the-center-of-the-neg, focal-length and ph-size. At some distance out from the center we should reach a value that isn't useful anymore (difference between exposure at the center and that point). Then it follows that there must be a minimum focal length for a certain negative size where you're using as much as possible of the negative size, aren't cutting the useful projected image circle and a shorter focal length would just give you a smaller circle on the paper. I assume that the thickness of the material that constitutes the ph matters, but I don't know to what degree. Lets also assume that there's a free line of sight of the ph from all points on the negative (no interference with whatever material the front is made off). So my question is, what's the equation for the combined light fall-off (my geometry is a little rusty :-)? And what would a reasonable cut-off value be (assume b&w photo paper as negative and I intend to scan it on a regular consumer grade scanner (Epson 1640))? If no-one has done this calculation, how about experience? Lets say I use 8x10 paper and cut it to 8x8. Have anyone done a short enough focal-length to get a round image of approx. 7-8" in diameter, what was your focal length? If I could find out the equation, then I could make a software center-filter to get even more useful info from the scan! Thanks in advance. Soren
