In starting to read "Post Exposure" by Ctein there is discussion of sharpness and of the possibility of some focus shifting due to UV exposure from tungston light sources when using some VC papers. It is suggested that using a Kodak 2B filter permanently in place will reduce UV exposure to some extent and reduce focus shift. Does anyone have any real world example of this? I would really like to reach a high degree of sharpness using my standard condensor type (tungston) head for 35mm enlargements and if my lens is of high quality and the enlarger aligned properly this seems like such a small adjustment to make. Any thoughts on this?

-- bill zelinski (willy226@yahoo.com), October 20, 1999

Answers

I personally add 5cc of yellow to my VC mix on my color enlarger because of this, I don't know if it is really necessary or not. Many top notch printers have always made a point of focusing in the final setup with all things in place and lens stopped down. VC paper threw a new thing into the mix, a color change. Much of the focus problems that people have had with VC papers is caused by this. Your grain magnifier is focused at a set distance, probably under white light. Adding a filter such as a strong magenta filter will make what seems to be a focus shift but what is really happening is that your eye will focus the different colors at different distances so the color shift has changed the focus distance for your eye in the grain magnifier not the focus from the negative to the lens to the paper. I don't know if I have explained this well. My suggestion is to focus before filtration and you won't have problems with these focus shift gremlins.

-- Jeff White (zonie@computer-concepts.com), October 20, 1999.

I've never had that problem. It would depend on the apochromatic properties of the lens, as well as the sensitivy of the paper to UV. Some enlargers already have a UV blocker built-in. It is easy to test to see whether you have this problem:

1. Focus as a accurately as possible (and check the focus when stopped down). Expose a sheet of paper.

2. Shift the focus a tiny fraction away from that in (1). Expose another sheet.

3. Repeat (2), in the opposite direction.

4. Now develop the 3 sheets.

Print (1) should be the sharpest, of course. If either of the others are, you have a problem. This test also gives you some information about alignment.

An alternative test: put a camera UV filter in the path. Does this improve the sharpness?

-- Alan Gibson (Alan.Gibson@technologist.com), October 20, 1999.

Stop down the lens two stops from wide open and you will have solved the erroneous problem anyway. Ansel didn't use any set-up with Ctein's ideas and he printed razor sharp images. And yes he did print with VC papers as they came out. And most enlarging lenses made since the 70's are apo's anyway. Just focus and then stop down two stops. James

-- james (james_mickelson@hotmail.com), October 21, 1999.

Ansel Adams printed with what was probably the first generation of vc papers, they were unaffected. The makers extended the sensitivity in attempts to widen the contrast range of the papers in successive generations. From what I've read the Agfa papers are the least affected. Stopping down lessens the problem at the expense of diffraction problems - not a solution.

-- Wayne DeWitt (wdewitt@snip.net), January 21, 2000.

Noticable diffraction doesn't set in till stopped down significantly. And no problems exist with dichroic heads or cold light heads so where is all of this problem coming from already. Sheesh! I see no degradation of print sharpness whatever I do to the filtration. james

-- Mr.Lumberjack (james_mickelson@hotmail.com), January 23, 2000.