Archived posting to the Leica Users Group, 2007/10/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]3. IR and Lenses ? In addition to the sensitivity of the sensor the quality of digtal infrared depends on the characteristic of the lens. For example, the Canon EF-S 18-55mm f/3.5-5.6 typically produces a pronounced hot spot as can be seen in the sample picture to the left. The hot spot is a a result of internal reflections within the lens produced by the lens' coatings. Some types of coating are not transparent to near-infrared wavelengths. The above was taken from a web site dedicated to Infra Red digital photography. Quite a few lenses are listed which produce these hot spots. The author did not include Leica lenses in the recommended or hot spot producing lists. Len On Oct 12, 2007, at 6:59 PM, Lottermoser George wrote: > Having attempted to deal with these "hot spotted" images in a > number of different ways; I think it would be a poor use of time to > attempt to correct it with blocking in the optical path. > > For a brief period I considered the possible use of a center > weighted filter. But once you see how "hard" this hot spot is; and > without contrast; it becomes fairly evident that any available > center filter would not work. You'd have to go through mind numbing > tests; looking at all sorts of various "donut" configurations in > the image to come anywhere close to a "block" correction. And even > if you could get there and correct for the "over exposure" you'd > still have to deal with the lack of contrast in that "spot." > > Regards, > George Lottermoser > george@imagist.com > > > > On Oct 12, 2007, at 5:45 PM, Jim Nichols wrote: > >> I've been following this thread, but since I have none of the >> equipment involved, haven't said anything. In looking at Tom's >> comment, it occurred to me that it might be interesting to try a >> blocking center spot applied to the filter, similar to the >> approach used with one of the early Leica lenses. > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information