Archived posting to the Leica Users Group, 1997/09/29
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 07:53 PM 9/28/97 -0800, you wrote: > >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 >Kauai Hawaii (still with some non-working keys on laptop--sorry) > Hope everything else works OK... I believe the blue filter trick is as follows: Nearly all of the really good enlarging lenses are f/2.8. This is necessary for critically sharp focus as it eliminates depth of field. If you use a 2.8 enlarging lens wide open with a grain focusser, the image is too bright. Hard to really critically focus. If you stop down to reduce the intensity, you now are dealing with depth of field and cannot really get the exact image plane critically sharp. By using a blue (monochrome viewing filter, Kodak Wratten #90) gel in your focusser, you cut the light intensity and build contrast in the grain image you are viewing. Easier to focus critically sharp. On some enlargers, the #90 is too dark. Use a level of CC-BLUE that works for you. Some enlarger heads, such as the ZBE Starlight head, can put out enough light to drill a hole in your eye via the focusser. Almost needs two #90 filters. Jim