Split Grade Printing Demystified

If you are printing on variable contrast (aka VC or multi-grade) paper (and let’s face it, who isn’t nowadays), you have almost certainly come across a mention of split grade printing; there tends to be a bit of lore and mythos around it, so it’s perhaps worth a brief explanation of what it actually does, because that seems to be widely missing and understanding it makes all the difference.

Once you know how split grade printing works, you will appreciate that it is a very useful technique when printing using traditional filters, one that it would be foolish not to leverage (but also that you can achieve the same in a much less awkward and entirely quantifiable way if you are printing using a variable contrast enlarger head, but I am getting ahead here).

The variable contrast paper is (function-wise) a very simple device: it has two emulsions on it, one which is only sensitive to blue light, and which has a very steep contrast curve (corresponding to a hard grade 5), while the other emulsion is only sensitive to green light, and has a very shallow contrast curve (corresponding to a very soft grade 00).

The actual image on the VC paper then arises as a superposition of the two emulsions, with its overall contrast curve being function of the relative proportions of the blue and green light used to expose it.

The traditional way of controlling the blue and green mix is to use filters, that, depending on the enlarger, go either between the light and the negative, or under the lens (the filters obscure the fact we are really working only with blue and green light because they don’t filter out the red component, which photographic paper is not sensitive to, but nevertheless that’s all that’s happening).

Now, the filters are only available in 1/2 grade increments, which is a shame because 1/2 a grade is a rather corse step, and in principle the contrast of the VC paper can be controlled with an infinite precision.

And that’s where split grade printing comes in: instead of a single exposure, we use two exposures at different grades to reach a contrast point somewhere between the 1/2 grade points provided by the filters; let me just repeat that:

Split grade printing is a way of hitting a grade on the paper that lies between the 1/2 grade steps provided by the filters (and that’s all that it is).

This is pretty obvious once you stop to think about it. Say we expose our print at grade 2 and then again at grade 4, this amounts to:

    e1 * B + e2 * G (the G2 exposure)
    e3 * B + e4 * G (the G4 exposure)

where B and G stand for the blue and green light, and e1, e2, e3, e4 are the respective exposures (with the e1/e2 and e3/e4 ratios mapping to grades 2 and 4 respectively).

But this is, of course, exactly equivalent to a single exposure of

    (e1 + e3) * B + (e2 + e4) * G

And voila, the split magic is gone, for this is just a point somewhere on the VC paper grade curve between 00 and 5, given by the ratio (e1 + e3)/(e2 + e4).

The basic problem with this technique is that it is nigh impossible to quantify the grade we are using, for, given a single exposure

    e1 * B + e2 * G,

there is an infinite number of possible combinations of two exposures to get to it, so when we are combining two exposures we never entirely know what grade we are actually printing at. And if we just try to randomly combine two different grade values it becomes an impossible to control mess very fast.

But we can make things a great deal simpler for ourselves by making the secondary exposure to use only one colour, i.e., using either grade 00 or grade 5 filter — in the former case, we will be reducing the grade provided by the primary exposure, while in the latter we will be increasing it; how much will depend on how long that secondary exposure is.

Of course, if you have access to a variable contrast enlarger, like the old Ilford system, or a more modern LED one (like that made by Heiland), split grade printing becomes fairly meaningless, because you can simply fine-tune the grade using the enlarger controls, which is much quicker, and is quantifiable, i.e., you know you are printing at grade 3.7 rather that a bit more than 3 (without realising it is in fact more than 3.5).

Plus modern contrast head gets around the problem of there not being enough blue light in the traditional enlarger light source, which necessitates the annoying exposure compensation when printing using the grades 4 and 5 filters — if you have access to a variable contrast enlarger, make most of it, for it’s a great tool.