RE: STARTING POINT
Don, > Ctein reports in Post Exposure that some vc emulsions are extraordinarily sensitive to the uv end of the spectrum which the human eye can't see to focus. > As I recall Ctein was probably the only person that could detect the focusing shifts he reported. Or to put it another way I could never discover or replicate the focus shift in my darkroom. Patrick Gainer posted the follow message on Photo.net, Sept. 19, 2001; 'There was an article in Photo Techniques some time ago (Jan/Feb 1997)called "Hazards of the Grain Focuser" written by some crazy guy named Pat Gainer. He proved by a series of tests that it is best when using a focusing device where an aerial image is made to coincide with a crosshair to use white or green light for focusing. The chromatic aberration of the eye is the culprit. Dig up the article for a complete description of the experiments. Using a deep blue filter for focusing resulted in an average error of about 9 mm in the direction reported by Ctein.' So Ctein's conclusion has been suspect for various reasons. A simple test with some RC paper should reveal the truth about the presumed UV focus shift. Some individuals that contact print large format negatives report that contact prints made on graded paper are sharper than contact prints made on RC paper. Again I've never seen the difference. I think Edmund Scientific at one time sold a film test targets that could be used to contact print onto film to test the resolving capability of a particular film. Perhaps the same approach could be used to compare the relative sharpness of VC papers and graded papers. Recently Jim Galli posted a comparison of different antique lenses using Kodak Polycontrast paper on his web site. Take a look at his shots, he never mentions focus shifts but perhaps he wasn't being critical enough. http://tonopahpictures.0catch.com/Apricot_Blossums/Apricot_Blossums.html Don Bryant
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