Our previous column described some of the idiosyncrasies encountered when using discrete variable-contrast (VC) filters to manage contrast with black-and-white VC papers. There are alternatives to those filters, notably vari- able-contrast enlarger heads and color heads. But, as described here, these too do not lack for “watch-outs.”
VC heads typically incorporate a pair of lamps, each behind a dichroic filter. The filters may be a magenta and yellow pair, or a blue and green pair. Pick a desired paper-grade number from the unit’s controller box and it will select the right mix of light from the two to produce that grade number. Additionally, the controllers usually adjust the voltage to each lamp so that overall print exposure times remain constant, independent of the grade number selected. As an example of a VC head, we evaluated an Ilford Multigrade 500H unit, using the same Ilford Multigrade IV Deluxe paper selected for last issue’s filter tests. And as before, we offer Figures 1 and 2 to illustrate how actual paper grade and printing time vary as the head’s grade number buttons are changed.
The results are indeed impressive: A wide range of rather evenly spaced contrasts, and “speed matching” that far surpasses the behavior of discrete filters.
Is there a downside to such a head? We will offer one note of caution for readers who may be considering such a purchase: VC papers “see” the colors blue and green just fine. Humans do not. A projected image that is intensely blue (the higher paper grade numbers) is particularly difficult for us to see, even with the darkroom’s safelights turned off. Given a choice, we find it much easier to print with VC heads that mix magenta and yellow light than those like the 500H that rely on blue and green.
The other option, a color-head enlarger, features continuously adjustable dials that insert any desired amount of yellow or magenta filtration into an otherwise white light source. Figures 3 and 4 demonstrate the behavior of a DeVere 504 color head.
The data “points” may be a bit misleading. They reflect the actual filter settings at which we chose to make measurements, but recall the settings are continuously variable; hence, so too are the contrast grade responses attainable. The obvious downside here is the total lack of speed matching: Every filter change requires a printing time adjustment. There are schemes in the literature for mixing the magenta and yellow, some even incorporating the cyan filter, which permit contrast changes while keeping exposure times constant. Such schemes effectively introduce varying amounts of neutral density into all but the otherwise longest exposure. While they accomplish their goal of providing speed matching, the price is a painfully long exposure for each and every print ever made. Of course if you go to the trouble of creating the equivalent of Figure 4 for your own color head, it is easy enough to look up how to change exposure time to correct for a filter change.
Lastly it’s important to note there are a couple things that routinely go wrong with both VC and color heads. The light from the lamps passes through the selected filters into a “mixing chamber” from which it exits via the lens. These mixing chambers are white inside when the lamphouse is new, but they invariably yellow over time. The effect is the same as permanently installing a yellow filter in the light path: High contrasts are simply unattainable. Also, magenta filters are prone to degrade over time and produce the same result—an enlarger no longer capable of a contrast harder than grade 3 or maybe 4. Either problem is easily remedied, but only if you know to look for it.