In practice is it really true that filter grades 0 through 3.5 require constant exposure, and then filter grades 4 through 5 require double? Is the transition really that abrupt (one stop from 3.5 to 4)? Does this have something to do with the two sensitive layers in the paper, and maybe the grade 4-5 layer is below the 0-3 layer so you really do need to abruptly add one stop to turn on the bottom layer?

-- Bill Akstens (bill.akstens@trw.com), April 05, 2001

Answers

Yes it's true. It's not an abrupt change in the paper. It's because the VC filters come in discrete steps. There is a #3.5 filter and a #4 filter that is considerably denser. The lower number filters have yellow and magenta density blended to keep constant exposure. The #4 and #5 are a different blend. If you use an enlarger with a VC or dichroic head contrast can be changed steplessly, with or without constant exposure.

-- Tim Brown (brownt@flash.net), April 05, 2001.

In our labs we have found that doubling per instructions is too much. Usually start by adding a 1/3 more exposure and move up to 1/2 more. Another interesting little item. try printing with a grade 2 filter and then without a filter at all. Manufactor indicates paper is grade 2 without filtration. They don't look the same in our lab. Occasionally, the time factor varies even from 3 to 3 1/2. However, this variable may have something to do with the age of the filters, age of light sources, etc. It is always a mystery. but fun!

-- Ann Clancy (aclancy@broadband.att.com), April 05, 2001.

Hi, Bill. First: VC paper incorporates different silver salts, each one sensitive to a specific wavelenght spam, all mixed together, not in layers as you refer. The question of sensibility of the compound so depends on filtering and qualities of light source. When they say exposure may be the same for all grades from 0 to 3.5, it only works for some medium grays, as you can easily notice when printing for shadows with different grades. When one gets to grades higher than 4, most of the emulsion response to green light is put aside and, for sure, the compound sensibility drops respectively. In practice, anyway, working with these short exposure scales always leads to accurate testing, as small changes in exposure show large paper response. You don't need to care so much on wich's the stated sensibility. Good work. Cesar B.

-- Cesar Barreto (cesarb@infolink.com.br), April 05, 2001.

As Tim said, it's the lower overall density of the grade 5 filter that's the reason for the big exposure step, not the paper emulsion.
The brand of VC paper makes a big difference to how much of an exposure change is needed, and the range of contrast you can get from any particular set of VC filters. You need to test the paper with your own enlarging setup, and take the maker's guide as just that, a guide.

-- Pete Andrews (p.l.andrews@bham.ac.uk), April 06, 2001.

This constant exposure is figured for medium gray. Generally speaking, I would not reccomend that you judge your prints by medium gray. The old idea of expose for the shadows and develop for the highlights when making negatives is also true for making prints, only in reverse. Find the correct exposure for your important highlights and then judge your shadows. If the shadows are too light then increase contrast, if the shadows are too dark then decrease contrast. You will find that to get your highlights the same at different contrasts an exposure change will be necessary. I have taken about a dozen printing workshops from some of the best printers working today and they all teach this basic approach to exposure and contrast.

-- Jeff White (jeff@jeffsphotos.com), April 06, 2001.