Let me state at the start that in the following discussion I am assuming the use of good quality negatives and variable contrast (VC) printing paper. If you are not yet producing consistently good negatives then spend time sorting that aspect out first (see the methods described on the Zone System pages).

Good printing has always been a three step process:

1. Find the correct exposure for the negative/paper combination

2. Match the paper contrast to the negative density range (usually done visually by test strips)

3. Apply local variations (burning and dodging)

This is the most efficient method in terms of both time and materials of achieving excellence in printing. However, many people attempt to rush matters and try to complete step two before they have established the best exposure (step 1). I believe the reason for this is that a change in paper contrast inevitably requires an adjustment to the exposure, hence many people (and many books) advocate trying to do steps 1 and 2 in one go. This usually results in neither step being done properly and hence the result is a disappointing print. What is required is a method that makes step one easier to complete accurately so people aren't tempted to skip straight to step two.

The tonal range (or contrast) of VC paper is controlled by the colour of the printing light. The colour of the light is controlled by filters. You can use either separate filters for each contrast grade (sold as kits) or use a colour/VC head on the enlarger. No matter which method you choose, using a filter reduces the amount of light reaching the paper which in turn changes the exposure (I am well aware that the filters in commercial sets are a combination of colours designed to produce similar exposures). It is the change of filter value that causes the exposure change thus requiring new exposure test strips to determine the 'corrected' exposure.

To remove the need to make new exposure test strips you can calibrate your filter system to determine accurate 'filter factors' that can then be used to calculate the new exposure time whenever you change contrast grades. I am aware that some filter kits have a calculator dial with them but this dial is usually based on the paper speed point (a middle paper tone of around 0.6 density) which is not the tone to use for determining print exposure in your darkroom. When you change contrast grade and then use these dials to correct the exposure, the mid-tone will be unchanged but all the lighter and darker tones will move which is not what you want to happen.

Getting your foot on a rock

Since the object of a filter change with VC paper is to change the tonal range of the paper, it is only possible to determine a filter factor that fixes the print value of one actual tone on the paper. The question is which tone do we use? Certainly in monochrome photography, many people will be familiar with the expression "expose for the important shadows and develop for the important high values" (I have added the word 'important' here). This refers to the best way to obtain good negative quality, it is a two stage process. My own equivalent expression for VC printing of negatives in the darkroom is "expose for the important high values and filter for the important shadows". Notice again that this is a two stage process. With film it would be a waste of time and materials to develop the film before you have exposed it in the camera. Similarly in the darkroom, it is a waste of time and materials to mess about with the contrast before you have nailed down the basic exposure time for the important lighter tones of the subject. The best exposure to use is the minimum that produces the high values of the print accurately. Get one foot on a rock before swinging the other around!

Following on from the above, since it is the exposure we want to correct when changing filters it makes sense to obtain filter factors based on a light tone (dense area on the negative). My personal preference is to use zone VIII as my pivot point. I make a zone VIII negative by exposing a lightly textured board, such as painted white fibre-board, three stops more than the light meter indicates and develop normally (this assumes your negative technique has been calibrated).

The filter calibration method is quite simple, set the enlarger up to produce a 10x8 inch image from the test negative. Set the lens aperture to the optimum f/No. Now start without any filters and make exposure test strips to find the exposure time that will produce the tone you want from your test negative. For a zone VIII negative area this would be very, very light grey with subtle detail showing (e.g. a white woollen sweater, snow etc.). This is a crucial step so don't rush it. When you have the correct time, make a test piece (about 2 inches square) at that exposure then process and dry it. This will be used as your reference or 'master' tone for each filter test. Make a note of this exposure time and f/No (try to use the same f/No throughout the tests).

Now using the first filter value, e.g. Grade 0 or 10Y, repeat the above test strip method until you find the exposure time that produces the same tone as your master piece. Make sure you dry each test before comparing them so that the dry-down effect is eliminated.
To find the factor for that filter value, simply divide the exposure time with the filter by the exposure time without the filter. For example, if your unfiltered time is 14 seconds and the time for 10Y is 15.4, the factor for 10Y is 15.4/14=1.1. Make a note of this factor.

Repeat this procedure for each filter value you wish to test. With a filter kit, you need to test each separate filter. With a colour head I recommend working in units of 10 for both the yellow and magenta filters. Needless to say, with a colour head there is more work to do but the reward is that you have finer control over your printing and you will save many hours of time in the future. A few hours invested now will be well repaid.

Eventually you will end up with a table like the following:

A working example

Let's say you start with a new negative and make test strips based on the lightest important tone. Assume this gives an exposure time of 12 seconds. You make a work print and decide the dark tones are not dark enough for your visualisation of this print. The print contrast is too low so you dial in some magenta filtration. Let's say you use 20M for the next test. Previously, you needed to make completely new test strips but using the filter factors this is not necessary. In this case, the new exposure time is 12seconds x 1.32 (the factor for 20M) which is 15.84 seconds (rounded to 15.8). When you make a test print at this exposure time you will find that the lightest tone has stayed where you want it but all the other tones have darkened proportionally hence increasing the print contrast.
You evaluate the new print and decide that the dark tones are now too dark. Too much contrast! The next test will be at 10M. Using the unfiltered time again, the new time is 12 seconds x 1.15 (the factor for 10M) which is 13.8 seconds.

If you prefer to start printing with a filter already in place, e.g. the grade 2 filter, the calculation required when changing filters is as follows. The new exposure time is new factor/old factor multiplied by the latest exposure time. In this case, had we started with 10M and obtained a time of 15 seconds for the first test, changing to 20M would produce the following 1.32/1.15 x 15 seconds. Which is 17.2 seconds.

If your evaluation of the original calibration tests (to produce the filter factors) wasn't as accurate as it should have been, you will notice that the key light tone of your print changes slightly as you change filtration. This indicates that the factor needs adjustment. If the tone gets darker the factor is too high, conversely if the tone gets lighter the factor is too low. You can fine-tune the factors as you gain experience with the method during printing sessions.

This kind of calibration may seem daunting but believe me it is a lot quicker than making new test strips every time you change filters and a calculator makes it easy! You will also save money and time in the future!