karl Shah-Jenner <shahjen@xxxxxxxxxxxx> writes: > 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 Cool idea. A bit crowded; I wish that the red/green/blue curves were in their own color, to make them easier to distinguish. > 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! :-) When I go from 50mm on the left axis horizontally right until I hit the blue curve, the intersection seems to me to be above about 550 on the f-stop scale. So am I misunderstanding the instructions, or the graph? I've done a tiny bit of pinhole photography with my Fuji S2. It's not actually auto-exposure, but having immediate feedback on the LCD (with histogram display!) is nearly as good, better in some ways. I could put the pinhole body cap on my PB4 bellows, mount that on my S2, and have a zoom digital pinhole system. What I *actually* hope to get to some day is putting a pinhole in a lensboard on the 4x5. Also zoom, but much bigger negative area. I want sharpness out of my pinhole, so I need the big negative. -- David Dyer-Bennet, <mailto:dd-b@xxxxxxxx>, <http://www.dd-b.net/dd-b/> RKBA: <http://noguns-nomoney.com> <http://www.dd-b.net/carry/> Photos: <dd-b.lighthunters.net> Snapshots: <www.dd-b.net/dd-b/SnapshotAlbum/> Dragaera/Steven Brust: <http://dragaera.info/>
