What is "Split-Grade Printing"?
“Split-Grade Printing” is the name given to a technique of exposing variable contrast black and white paper using two exposures instead of a single one. One exposure is made through a soft filter (usually grade 0) and the other through a hard one (usually grade 5). The ratio of the two exposures determines the contrast of the print, and the overall length of the combined exposures determines its lightness or darkness. The technique offers smooth and more or less infinite control of contrast between the soft and hard limits but requires only two filters. It is the system employed by the StopClock 500, the Analyser 500 and the automated Heiland SplitGrade controller, all of which require a special enlarger head. Of course, variable contrast enlarger heads (and to a lesser extent colour heads) also allow smooth contrast control, but the advantage of the split-grade system is that it is easier to determine the correct exposure and contrast if you don’t have an exposure meter. Also, dodging and burning during the two exposures can control the highlights and shadows separately offering a high degree of local contrast control. The technique can be used on any enlarger that allows the use of above- or below-lens filters, or that uses a colour or VC head with built-in filters.
The StopClock’s Split-Grade Mode.
We have incorporated an enhanced split-grade mode into the latest StopClock Professional and Stop Clock Vario. Previously the split-grade mode simply locked the two channels together and displayed an equivalent paper grade, after which altering the exposure time in either channel caused the other to track it, thereby maintaining print contrast if the exposure was changed. With the new version, this only happens if channel one is changed; channel two can be adjusted independently. Providing that the channel one exposure is made through the soft filter and the channel two one through the hard filter, this allows contrast to be controlled using the channel two exposure only, and exposure to be controlled using channel one only. Hence, you have independent and continuous control of exposure and contrast!
Adjustments to the exposure will probably be required for contrast changes at the extremes of the range, but over the middle range of grades 1-3 this adjustment is minimal. To get the best from the method it is important to base your exposure only on the highlights; once the highlight exposure is established (by means of a test strip or one of our exposure meters) you can then add the hard exposure and adjust the shadows by varying the length of the second exposure. Please note that for the StopClock’s “equivalent paper grade” indication to be meaningful you need to be using Ilford Multigrade filters which are speed matched. Other filters, or colour or VC enlargers, may result in a different contrast level than that for each nominal grade. However, the system will still work.
Equal exposures through each filter produces a contrast equivalent to grade 2 (if Ilford Multigrade filters are in use). Thereafter, increasing the hard exposure by a stop will increase contrast by one grade, similarly decreasing it by half a stop will reduce contrast by half a grade. This technique should work with any variable contrast paper, and produce smooth and continuous control of contrast over a wide range with evenly spaced “grades”.
The Split Grade technique has been popularised by expert UK photographer and printer Les McLean and there is an introductory article available on his web site here. More information on Split-Grade Printing is available in this article (pdf file) by Chris Woodhouse which is an extract from the book “Way Beyond Monochrome”, currently out of print but the second edition is expected soon.
Using the older StopClock Professional for Split-Grade Printing.
Owners of the StopClock Professional with v8.3 firmware or earlier can still enjoy the benefits of split-grade printing as outlined above. First of all determine the soft exposure based on the highlights. Then switch to channel 2 for the hard exposure and determine that in the usual way. Press and hold the “Split Grade” button to display the equivalent paper grade. You can then adjust the overall exposure using channel 1, and channel 2 will track it to maintain contrast. If you want to adjust the contrast only, press and hold “Split Grade” to exit that mode and adjust channel 2 only. Press and hold “Split Grade” once again to display the grade and re-lock the channels together.