Archived posting to the Leica Users Group, 2000/02/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>It would be nice to know what is "fact" and what is "common knowlege" but >not true. I've also heard that back scratches are a problem. I have a >Summarit with a scratch (just one) on the back and I can't see that the >image is harmed in any way. > >Ken Wilcox > >At 5:23 -0500 2/15/0, a fine scholar, RGKEG@aol.com wrote: > >>My .02. >> >>When I was shooting motion picture film those gel holders for behind the lens >>filtration were avoided at all costs because any marks at the back were way >>worse than marks at the front. >> >>I was also taught in no uncertain terms that front scratches can be lived >>with, but lenses with rear element scratches should be avoided. Slight >>cleaning marks might be another thing, though. >> >>Sorry, never tested it :) >> >> Ron Kutak > > >---- >Ken Wilcox Carolyn's Personal Touch Portraits > preferred---> <wilcox@tir.com> > <kwilcox@gfn.org> Back scratches are more problematic. The reason is that while most image rays are fairly parallel before they hit the lens, and are collected evenly over an area equal to the apparent aperture as seen from the front, the rays travelling from the back of the lens are converging quite strongly toward any given spot on the film, and pass through a much smaller area on the back of the lens (unless we're talking serious macro). Therefore if there is a serious scratch or defect on the rear element, it will block or interfere with a larger percentage of the image forming light in the worst case than a similar sized scratch on the front. Evenly spread cleaning marks would have about equal effect front or back. BTW, cleaning marks on the front of the lens are generally worse as far as having a detrimental effect on the picture than a scratch. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com