Archived posting to the Leica Users Group, 1998/02/05
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Jacques, A pinhole photograph, to my mind, produces a distortionless representation of the world because its focal length changes in relation to the distance any particular imaged point will travel. The distance from the pinhole to extreme film edge is farther than the center to film center pinhole distance and since focal length is a function of distance (the farther a pinhole is placed, the longer the effective focal length), its projection to a flat field guarantees seamlessly variable image magnification, from center to edge. I'm thinking that a dished focal plane would create a fisheye type image, provided that the dish were made so that all rays met it at equidistant angles. If that's true, then there is a 'built in' image magnification variable. Then again it also seems to prove that the distance travelled is the key to variations in magnification and not lens induced, variable field magnification. I'm, right now, playing with a 50mm Rollei lens. When its reversed and held so that my eye focusses on the focal plane, it seems to have more positive power in the center and less magnification at the edge. That's the opposite of what I was expecting but it doesn't seem constant in magnification across the image field (as I move my eye from lens edge to center). What do you see? Here's a quote from David Rosen, in a discussion 2 years ago on this subject: I believe you are actually discussing the effects of oblique projection, whereby the magnification increases as you move off-axis. True teles have more oblique projection toward the edges simply because they are close to the film than regular long focus lenses. Inverted teles, or retrofocus wideangles, being farther from the film than the merely widefield [but non-retro] wide lens will exhibit less oblique projection effects. You can reasonably think of it as the projected image keystoning toward the corners. This is usually noticed as the egghead effect. David Rosen golem@capital.net Somewhere on this hardrive, I have excerpts from a conversation I had with Harold Merklinger on the subject. If I find something to-the-point short enough to quote, I'll post that as well. Regards, Danny Gonzalez