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G R E E N M A N P H O T O G R A P H Y -- R o n H a m m o n d
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What is split filter printing?

Split filter printing is the sequential use of a high-contrast and a low-contrast filter to produce a print on variable-contrast paper. It is certainly a common and very useful technique to give the print a base exposure with one filter and then burn areas of the print with a different filter. Split filter printing produces similar results but with, in my opinion, better control and better ability to pre-visualize the results. I had used split filter printing quite a bit and found it both very frustrating and, occasionally, a very useful addition to my darkroom tool bag.

There are several approaches to making a split filter print. One suggests beginning with a test strip using the low contrast filter to determine the exposure to give just noticeable detail in the highest highlights. The second step is to make a full sheet exposure with that time and filter and then, superimposed on that exposure, make a test strip with the high contrast filter to determine what it takes to produce black. A second approach is to make a checkerboard test print with strips from top to bottom using the low contrast filter and strips from left to right with the high contrast filter.

I developed a third approach to split filter printing that allows me to use it as extension of my normal printing process. I always make a full frame work print with a grade 1 filter for each negative I intend to print. If I judge that dodging and burning will be difficult then I use this approach:

	Make a test print with a grade 5 filter to determine the exposure needed for a hard black.
	Estimate from the work print what grade filter will be appropriate (apart from dodging and burning)
	Look up the percentage of grade 5 exposure and grade 00 exposure for that filter grade
	Look up the total exposure time
	Make a print with this filter split and iterate as needed to zero in on the overall exposures for the print.
	Iterate as needed to zero in on the dodging and burning needed for the finished print.

The pdf file has all the details.

What is the problem?

There is a certain amount of folklore and mythology concerning split filter printing on variable contrast papers. I decided to do some testing to determine what is and is not possible with split filter printing. I was goaded into doing these experiments by protracted discussions with Paul Butzi, Bernice Halpern Cutler, and Joe Budne. I am grateful for their help and prodding.

One piece of folklore is that you cannot achieve a good black on variable contrast paper with a low-contrast filter. This is not the case. It is easy to see how this notion came to be, however, because modern variable contrast paper exposed with a low contrast filter has a very long shoulder and it is easy to underexpose enough that the paper does not yield a good black.

A second piece of folklore is that split filter printing allow you to shape the response curve of the paper in ways that cannot be achieved by a single filter. Alas, this is not true either. For a straight print; that is, no dodging or burning, any split filter print has an equivalent using only a single exposure. However, split filter printing can be a very useful darkroom tool:

	If you use discrete filters, split filter printing is one way to fill in the cracks between grades.
	Much more importantly, split filter printing combined with dodging and burning gives you a measure of control that is impossible to achieve with any other technique I know -- especially if you have only two hands.

I have already improved my printing by going through this exercise. It improved my understanding of variable contrast paper response and of split filter printing. I now understand what split filter printing can and cannot do and why.

I used my test data to determine how to calculate exposure times when changing variable contrast filters. This information turned out to be a useful side effect of the investigation of split filter printing. I posted the exposure factor table on my darkroom wall. This allows me to compute (within a quarter stop or so) the exposure change required to maintain a good black when changing filter grades.

I also posted the chart for converting a single filter exposure to a split filter exposure on my darkroom wall. I will use split filter printing more frequently than previously because I will be able to close in more quickly on the exposure and split for a given negative.

Don’t use my results!

For heaven’s sake, don’t take my results and expect them to fit your materials and darkroom process! I’m sure that you will see substantial variations with different brands of paper, filters, different developers and toners, different enlargers, different processing times. When I change any of the above I will grit my teeth and go through these experiments again.