Archived posting to the Leica Users Group, 1997/12/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Dan Post wrote: >I was trying to go to sleep- hence the late hour of this posting, but I kept >thinking about the problem of light fall off. >Then it struck me. I was taught that the image of a '50mm' lens focused at >infinity is the same size as the image formed by a pinhole 50mm from the >film plane. The compound lens acts the same way, even though the optical >center changes during focusing. If one imagines a line through the axis of >the pinhole, to the center of the film, and that the visual field of the >pinhole is illuminated evenly, then the light from the visual field will >form a 'cone' of light from the pinhole to the film. >If you further imagine that the light striking the film on the axis, >directly behind the pinhole, has an intensity of, say, one or unity, then >the light decreases as you move away from the axis as a function of the >cosine of the angle from the axis. This, of course, holds strictly if the >film plane is curved so that the distance from the center of the pinhole is >the same at the center of the film as it is at the edge. However, the film >is flat so the edges have a reduced illumination, not only due to the cosine >rule, but the law of inverse squares. It seems that the light fall off will >ALWAYS be greater in a wide angled lens from this simple fact. >Now what surprises me is that one can get such good pictures from wide angle >lenses, which must be a testament somewhat to the latitude of film. >Now I'm going to bed! I got this off my mind! Thanks for listening In optics these are part of the 'cos to the fourth' function that standard construction lenses obey in a general way. With a bit of fudging this can be compensated for to a small degree. If you want a lot better eveness of illumination, retrofocus lenses are the ticket. You get a lot of other problems, but eveness of illumination can be greatly improved. Telephoto designs make matters worse, by causing more falloff than the 'cos-fourth' law dictates, but it usually doesn't matter much because telephoto designs are used on long focal lengths, where the 'cos-fourth' falloff is small anyways. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com