A set of filters with which to control printing contrast is about as basic a tool in the black-and-white darkroom as a pair of print tongs or a thermometer. And that makes them very easy to take for granted. Pull out a number 3 filter and it’s obviously a 3—not a 21⁄2 or 31⁄2—definitely a 3. Or is it? The answer is, naturally, “it depends.”
Such filters have been around since the days of Du Pont’s Varigam system and are available to this day under perhaps a dozen brand names. We selected two of the better known, Ilford Multi- grade and Kodak Polymax filters. Several boxes of each were evaluated by printing step tablets through them and noting the actual paper-contrast grade obtained with each filter. Of course, it would be futile to conduct such a test with a paper incapable of achieving a full range of contrasts in the first place, and, as explained in our March/April 2005 PT report on variable-contrast papers, that is indeed the case for many products. The data we report here are for a certifiably full-range paper, the glossy version of Ilford Multigrade IV RC Deluxe paper. Virtually identical results were obtained with other well- designed papers, and for assorted surfaces.
So much for all filters being created equal. And, as noted, there are many other brands out there, any one of which may have its own unique pattern. The good news is box-to-box variability with each brand was nil: Manufacturing consistency within our small sample looks to be nonexistent (but see the closing caveat). So which of these is better? Both cover a wide range from a grade number of –1 to greater than 5. But the Ilford filters do this in a smoothly predictable fashion, incrementing in very equal steps from the flattest to the hardest response.
On the other hand, for a photographer whose negatives consistently print on “average” grades, Kodak’s filters offer smaller increments for better fine-tuning in the grade number 1⁄2 to 3 range. Fact is, for most of us there is something to be said for owning both sets.
But contrast response is not the whole story. Filters are also reputed to be “speed matched.” Switch from one filter to another and there is no need to adjust exposure time. At least up to a 31⁄2 filter. Those labeled 4 and above are claimed to be one stop slower (double the printing time) of the others in the box. Sounds good. Does it work? Speed matching is defined by the manufacturers as ensuring the same exposure time will produce a density of 0.60 above base plus fog with any filter up to and including the 31⁄2. Filters number 4 and above will require double that time.
Figures 3 and 4 denote actual exposure times when the number 2 filter is slaved to print in exactly 10 seconds. Ideally the number 4 and above would require 20 seconds and all others would match at 10 seconds.
The results are close enough to that performance to say the assumption of “speed matching” is at least a useful approximation.
One other—and rather horrifying— comment on filters: Among the several boxes we tested, it was apparent some of the individual filters had been incorrectly labeled. If you have never done so, it may be worth your while to lay your set out in sequence on a light box and confirm that they represent a visually smooth progression of colors. In fact, if you have been using the same filter set for a number of years, it might be worthwhile to take it a step further, print a step tablet with each and assess actual grade numbers for yourself. Filters, particularly those towards the magenta end, can fade over time and curtail the range of contrasts available to you.