Archived posting to the Leica Users Group, 2005/06/16[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
This is, in part, a retraction of my previous post. It is also a correction of an error. In describing the central portion of the lens, I named the wrong F stop. First, I want to say that my advanced physics degree is vintage 1951 and that I gave up physics for computer biz in 1958. So, regarding geometric optics, it took a while before old memories filtered back into consciousness. So, to simplify the complicated way I stated things earlier, trying to be accurate: a focus shift implies that the outer portions of a lens have a slightly different focal length than the central portion. OK, the correction for spherical aberration is supposed to prevent this. But, it doesn't. In a simple, one-element lens, the focal length changes monotonically as one goes from the center to the periphery. Now, aberration corrections are always a compromise. If there were no spherical aberration, a plot of focal length vs. distance from the lens center would be a horizontal straight line. The best correction that can be made is a curve that wiggles on both sides of this line. So, the average focal length of the central portion may differ from that of an outer portion. Now, why do we stop down when enlarging? The main reason is to minimize the effect of the various aberrations. A secondary reason is to lengthen the exposure for a) accuracy and b) to facilitate manipulation such as dodging and burning in. Now, having expounded on the nature of the spherical aberration correction, I can state a theorem: the loss in sharpness caused by focusing at maximum aperture and then stopping down will be less than the loss caused by making the enlargement at full aperture. -- Herbert Kanner email@example.com 650-326-8204 Do not meddle in the affairs of cats, for they are subtle and will pee on your computer!