I just put something up for my students and thought it may be of interest to others.. I have a really nice graph I put together showing optimum apertures at given focal lengths (or optimum focal lengths for given apertures) which relates apertures to effective apertures (f stops) - AND - (wait for it) it also specs the optimum aperture/focal length combinations for particular wavelengths oflight! All in a kewl modern-type curvy graph with coloured bits an' stuff. find it here now for a limited time only!: (the left vertical axis is the focal length in mm) http://tinyurl.com/392xr Someone then emailed back wanting to know how I'd come up with the graph, so here's the background - f=aperture L= focal length d=aperture diameter in mm w=wavelength blue w = 0.00044 green = 0.00050 red = 0.00065 IR = 0.00095 (wonder if I should have used 0.00075 instead?) f=l/d for the straight line bits (aperture marked in mm on the lines) optimum aperture (d that is) = square root of (1.22 x L x w) for those who want to use it, lets say you have a fixed focal length, non-zooming type pinhole camera - you measure the distance from the film plane to the pinhole, that's your focal length. next you'd look at the graph and work out what sized hole you want to use - lets say you want to shoot with ortho film (blue sensitive) and you're using a box with a 50mm focal length.. go across the 50mm line from the left and see where it smacks into the curvy-upy line for blue light and you'll find it intersects at a f number (go down to the bottom axis ) at around f 1020 - hey! how close to the proper real f number of f 1024 IS that! :-) looking around the graph for one of those radial lines eminating from the bottom left corner you'd see one passes really close to the the place where the curvy blue light line intersects the imaginary line you drew out from the 50mm focal length.. and this line is the 0.5mm aperture, so taking your industrial drill press and your 0.5mm drill bit, you make a hole in some thing sheet brass and bob's your auntie! you have a pinhole optimised for sharpness at your given focal length.. If you wanted to be really pedantic you could work out that you needed a 0.52681mm hole (or something like that) and you could try to find a 0.52681mm drill bit, but I don't like your chances :-/ Better to stick with a few sensible hole sizes and use the appropriate one for any given job - a bit like, um, using real lenses with glass in them! :-) as to making your exposure, well, knowing what the actual effective aperture is takes some of the guesswork out of exposing the film (duh!) so lets see - bright sunny day, 4ASA ortho film, effective aperture of 1024 .. so normally I'd be shooting this shot at around f16 @ 1/4 of a second, and I've got an aperture that's cutting the light by 12 stops more than f16 so I need to make the exposure 12 stops longer - like around 2048 seconds or 34 minutes and 8 seconds.. ish. Allow for reciprocity if needed, and any contrast correction you want to do by pushing/pulling then you're done :-) Of course, until they make a pinhole camera with a variable focal length (non-tele zoom ;-) and a variable aperture that can do autoexposures, you could either guess or use the kewl graph and a braincell or two :-) Lastly, don't forget to write down EVERYTHING you do when making the exposure, checking your calcs before making the shot - and then you'll have some frame of reference to turn to afterwards if something goes wrong. Isn't pinhole photography fun!? :-) .. I got sick of hand drawing it and microsoftXL gave me headaches so I found a nice little graphing program to make the pretty pic for me :-) karl
