Archived posting to the Leica Users Group, 1997/10/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>>Gene Nocon--a gentleman who runs lab across the street from studio and >>has printed for many greats uses a blue filter in his focusing tool >>Without it he says prints arent truly focused He showed me print >>coparisons and he is right Probably big factor at wider F stops >>Apparently this is standard knowleddge among high level lab types When >>I get back into do my own printing I will definitely get the >>filter maybe APO lenses dont need it I`ll ask when I return >>Donal Philby I've just got the book by Gene Nocon "Photographic printing" and in it he says; "Using the blue filter on the Omega Micro-grain focuser; at this enlargement the grain can be seen to be pin sharp on the print. The image projected onto the easel will actually appear out of focus after focusing with the blue filter, but the print will be sharply focused. Without the blue filter on the grain focuser the image appeared to be in focus. The resulting print, however, is out of focus." This suggests to me that it is not a question of using a blue filter to increase contrast or reduce brightness, but that the image that appears on the paper when printed is formed from predominantly blue light or light that is focusing in the same plane as blue light. Otherwise, the image in the above description that was focused without the blue filter would be in focus when developed! What is not clear is how this applies to VC paper. The answer, or some of it, may be in Ctein's Darkroom User article (Darkrrom User, 1996, #1) on the best enlarging lenses, where he says; "I recently discovered that there can be huge differences between the plane of best focus seen by our eyes and the proper focal plane for VC print paper. I've seen as much as a 15mm focusing error when printing 10x8's from my 35mm negatives, using my Beseler Dichro 45 color head, Computar 55mm f1.9 lens and Kodak Polymax paper. That shirt produces a print which is visibly unsharp! So far most other printers have not found a serious focus problem, but "most" is far from "all"! The cause is longitudinal chromatic aberration. Longitudinal chromatic aberration causes different colrs to be focused at different distances from the lens. As an image is moved in and out of best focus, the final detail will (for example) look magenta in hue on one side of the best focus distance and and green in hue on the other. Good enlarging lenses are adequately corrected over the red-green-blue part of the spectrum. Unfortunately modern VC papers respond well to much shorter wavelenghts of light, down into the near UV where the human eye sees almost nothing. As a result, even when we focus our enlargers through a deep blue filter, we're using a diffeent part of the spectrum than the print paper sees. The print paperresponds to light outside the normal spectral range for which the lens is optically corrected. Not all enlargers put out enough violet-UV to cause problems. Not all VC papers are sensitised this extremely. The quality of LCA correction for V-UV light varies greatly with the lens design. Consequently you may not have any problem, but that's far from a certainty.... ...I'll be reporting on this problem in much more detail in a few months time in this magazine. Meanwhile I suggest you run a careful focus check on any lens you're considering buying, if you print with VC papers. I'd look especially carefully at lenseswhich I report as having larger-than-average amounts of longitudinal chromatic aberration in the visible spectrum." Interestingly, in this article's top ten lenses test results, the Rodenstock Apo-Rodagon N 50mm/f2.8 has "worse than average red-green longitudinal colour." Fascinating... Joe Berenbaum