B&W Print Analyzer

Print Exposure Analyzer

Automated Contrast and Exposure Time

By Doug Criner

Like most amateurs, I use variable contrast paper for making black & white prints. There are three exposure adjustments to be made on the enlarger: the lens f-stop, the exposure time, and the contrast-grade selection. The customary approach is by trial and error, using test strips to find the combination that produces the desired print.

To help add a little science to the process, I invested in a Heiland Splitgrade system. This is essentially a special-purpose microcontroller that measures the range of tonal values in the negative and then recommends an exposure time and contrast setting. To use the system, you focus the negative image onto the printing easel, and then slide a mouse-like light probe around within the image. The system then knows the range of values in the image and computes the exposure time and contrast grade to produce a full range of values in the print. The system has built-in characteristic curve data for a variety of printing papers.

I use the Splitgrade system in a "manual" mode–in other words, I read the recommended exposure time and contrast grade on the Splitgrade's display, and then set my Time-O-Lite timer and Beseler variable contrast head accordingly. For some enlargers (not mine), it is possible install a filter module that is actually controlled by the Splitgrade itself. This doesn't seem like a major advantage to me–it takes about one second for me to adjust the contrast knob on my enlarger's head.

The Splitgrade system also can serve as an enlarger timer, making two exposures, one at pure yellow and one at pure magenta. My enlarger provides a continuously variable mixture between yellow and magenta, so I don't see the point of making two separate exposures to get the same mixture. I just make one exposure at the recommended contrast setting.

Maybe it's just an indictment of what I was doing before, but my results with the Splitgrade system have been spectacular.

How it Works

The system is essentially a microcontroller with proprietary firmware. Based on my experience with the system, I have developed a pretty good idea how it works.

The Splitgrade controller has built into its firmware the characteristic curves, or perhaps just the toe and shoulder locations of the curves, for a variety of popular papers. Since I purchased my Splitgrade, I have received from Heiland, in Germany, one update of the paper data, which was easy to install into the system.

Passing the mouse-like sensor over the unfiltered image projected by the enlarger, the Splitgrade measures the illuminance of the darkest and lightest areas of the negative image, and then computes the contrast grade and exposure time required to print the brightest highlight as essentially white and the deepest shadow as essentially black.

This will result in the print having a full range of values–from white to black. For the 95% of my negatives which should be printed with a wide range of exposure values, particularly landscapes, street scenes, architectural views, or still lifes, this will result in pleasing prints, or at least very close to what I want. (True, I could use an inexpensive light meter to measure the range of exposure values in the image projected onto the paper easel, and from that guess at the required contrast grade to achieve a full range of values in the print. But I would still have to make a series of test strips to then determine the appropriate exposure time.)

For example, imagine a negative with just two exposure areas--one, say, at Zone IV and another at Zone VI. The Splitgrade will print the negative with just two values, but one will be essentially black and the other will be white. This can lead to some problems. For example, I have a negative (not one of my better shots) of a cathedral in Mexico, taken in dim, overcast conditions, where the church in the foreground is all essentially at Zone IV and the sky is at perhaps Zone VII. Using the Splitgrade's recommended print exposure, the church winds up as a pure black silhouette against a white sky.

The Splitgrade doesn't necessarily do well with portraits–many full-face, Caucasian portraits need to be printed somewhat flat rather than contrasty. If faces are an important element of a photo, I tend to reduce the Splitgrade's recommended contrast grade by ½ to one grade and to reduce the exposure time by ½ to one stop. This usually is a good starting point. Another solution is to use an accessory developed by Heiland, the "Splitgrade-Comfort Control Interface."

The Comfort interface is an accessory to the main Splitgrade controller. (The "comfort" appellation seems a little strange, perhaps the result of a literal English translation from the German.) Basically, the Comfort interface, which connects to the main Splitgrade controller through a cable, provides some improved displays and switches, which are convenient. More importantly, the Comfort permits one to measure the zones of the various mid-tones in the picture. Thus, it is easy to place a person's face on Zone VI by adjusting the base exposure time or contrast from that recommended by the Splitgrade.

For more information on the Splitgrade system, see Heiland's website at www.heilandelectronic.de and their U.S. representative, Versalab, at www.versalab.com

Here is a picture of the Splitgrade system that I "lifted" off the Heiland website:

The largest item is the main Splitgrade controller module, housed in a rugged, Teutonic-looking steel case, with a gray hammer-tone finish–obviously not made in Asia. The white device is the light intensity detector which is skimmed around the image projected on the easel. The black item is a footswitch to turn on the enlarger light; I don't use the footswitch since I use a separate timer for my enlarger.

The Comfort interface is similar in appearance to the main Splitgrade controller, although smaller.

Fine-Tuning VC Prints

Now, once you've made the first print of a negative using the Splitgrade's recommended settings, you may want to fine tune it. Rather than getting bogged down with how variable contrast papers work, there are really just two things to remember.

To adjust the highlights, i.e., the white areas of the print, increase or decrease the exposure time. To adjust the shadow areas, the dark areas of the print, increase or decrease the contrast setting.

As an example, consider this picture of a covered bridge in snow:

In this photo, the lightest area is the snow. If the snow is too dark, reduce the exposure time. If the snow is too white and lacks texture, increase exposure time. Either way, the darkest areas will be relatively unaffected. (If you can't obtain texture in the snow, then examine the negative with a loupe–it may be overexposed.)

The darkest areas are the tree trunks and the entry into the bridge. To lighten these, reduce contrast. To darken, increase contrast. Whichever, the snow will stay pretty much the same.

Well, you ask, what about the mid-tones? For example, it may be important to lighten or darken the siding of the bridge itself. You then have two choices: tinker with the exposure time and also change the snow, or change the contrast setting and possibly change the tonal value of the trees and the entrance into the bridge. You can't have your cake and eat it, too.

Well, yes you can. If you insist that you want to adjust the siding of the bridge, and not change anything else, then pal (as we say in Chicago), you're into the dodging and/or burning business.