Archived posting to the Leica Users Group, 1997/12/04
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 11:00 PM 12/3/97 -0600, Eric Welch wrote: >As far as I know, Leica has only two diffraction-limited lenses. So what >good are those numbers? I think your question assumes that if a lens isn't diffraction-limited, then diffraction doesn't matter. This is a "weakest link" view of how a photographic system works -- the weakest link in a chain fails first. But that isn't how photographic systems work -- instead, the loss of sharpness is cumulative. I gave the formula for the limit of resolution of a diffraction limited lens, but that very same amount of diffraction (it's the diameter of the Airy disk) is present whether or not the lens is diffraction-limited, and the blurring it causes will further degrade the image produced by a soft lens. > As I heard it, the angle of light in a >wide angle lens is much more severe (light passing over the edge of the >aperture). And thus diffraction is increased compared to the light >travelling through a telephoto where the light is diffracted less becasue >the angle of attack on the edge of the aperture blade was less severe. This is getting kind of complicated. The light doesn't "know" the angle at which it strikes an aperture edge. If we think about diffraction in the way that classical wave theory presents it (which is the only way I learned, although there are more complicated and accurate ways), each point on the wavefront approaching an edge is a little point emitter. It emits light sideways as well as forwards. But the light from the points on the sides of each point constructively interfere with the sideways light and it all adds up to the wavefront. When you pass an obstruction, the points that are blocked by the obstruction are taken away. The little point emitters that were next to them continue to emit their secondary waves. So do those close to the edge. The light from these point emitters spreads sideways (in all directions, strictly speaking) and the wavefronts interfere in ways that give you the nodal irradiance pattern characteristic of diffraction. Nothing about the angle of the light and the aperture edge is involved. The irradiance pattern's image on film or a screen it's projected on will vary according to the angle of the screen or the film plane to the wavefront, of course, but I don't think that effect comes into play here. I'm not a lens designer. I don't know anything about designing wide-angle lenses, except for a few very simple ones. I'm not even a physicist. But I have measured aperture diffraction patterns -- it was one of those stupid labs I got to do when I took classical wave theory. The patterns do match up pretty well with the theory. >So please, define what diffraction limited means. Seems to me these numbers >don't make sense. A "diffraction-limited" optical system is one in which aberrations are so small that the primary factor affecting image quality is the aperture diffraction. Why do you say the numbers don't make sense? Have you observed diffraction being a problem in short-focal-length lenses and not in long-focal-length lenses at the same aperture? If so, how were you able to tell that diffraction was responsible for the effects you saw? - -Patrick