For some prints, especially when using a large aperture (f4 or f5.6) I need some neutral density to increase exposure times, to do some succesful dodging and burning. This can easily be accomplished when dialing in some neutral gray from the dichroic colorhead of my Durst 670 enlarger. Used to think in f-stops, I like to dial in f-stops. I know there is a relation between f-stops and dichroic units, but have no clue what exactly. The units range from 0 to 170.

thx, Marc.

-- Marc Leest (mmm@n2photography.com), December 12, 2001

Answers

Greetings,

I think Durst dichro units are not the same as Kodak's et al., so you'll need to check, but essentially, adding 30 units of cyan, magenta & yellow is the same as adding 1 stop of neutral density. Each 30 units equals 1 stop, so if you need two stop of ND, just dial in 60 units of C,M & Y.

Regards, Pete

-- Pete Caluori (pcaluori@hotmail.com), December 12, 2001.

I agree that the amount of ND filtration to equal exactly one or two f stops is going to vary somewhat enlarger to enlarger. If you have an enlarging photometer you can easily find the difference by calibrating the deviation of the meter with one stop increase or decrease in light using your lens. Then aim for the same deviation using using dichro filtration. You can also use a light meter in the same manner. Another simple method to decrease light, which does not tie up your dichro filters, is to add extra diffusion material,(one stops worth),under your mixing chamber. It comes in different thickness so you can make up just enough to alter the illumination by whatever whole stop you desire. Even simpler of course is to just stop down your lens another stop or two. If it is a high qualty lens, although it may look slightly less sharp through a grain focuser, you will not see the difference with the naked eye.

-- Arden Howell (serenisea@aol.com), December 13, 2001.

With B&W paper, the Cyan filter does nothing, since it's minus red. You only need to use the Magenta and Yellow filters in equal quantities. 30m + 30y for each stop, approximately.

-- Pete Andrews (p.l.andrews@bham.ac.uk), December 13, 2001.

Check Ed Buffaloe's site www.unblikingeye.com and look for the article on using f stops for printing, is very useful and does make a lot of sense.

-- Jorge Gasteazoro (jorgegm58@prodigy.net.mx), December 13, 2001.