This method works on the principle of neutral density filters and exposure times. For example, if a print required 10 seconds exposure and you placed a 0.3 (one stop) neutral density filter in the light source the print would then need 20 seconds exposure. If you used a 0.1 ND filter the exposure time would need to be increased by one third of a stop, i.e. the new time would be approximately 12.6 seconds.

Using this principle of exposure changes with density, to obtain the SAME print tone from a set of zone test negatives that have different densities, i.e. zone I, zone II, zone III etc., the print from each test negative will require a different exposure time (the enlarger aperture MUST remain constant!). Each of these exposure times can be divided by a reference start time (see below) and a simple calculation made that will convert this ratio of exposure times to a density value. The procedure is as follows.

1. The first step is to produce a reference tone. Make a contact print of the blank film using an exposure time of e.g. 10 seconds (you can use any time but 10 makes the calculations easier later). Adjust the enlarger height and lens aperture so that you obtain a mid-grey print tone at this exposure time (this may need a couple of test pieces). Don't make the print tone too dark nor too light, a Kodak Gray card is a useful visual target tone. This exposure time and print tone are your reference time and reference tone.

2. Using a zone test negative, make a contact print. Assuming the negative has more density than the blank film, the exposure time for this print will be more than your reference time. DO NOT CHANGE THE LENS APERTURE OR ENLARGER HEIGHT! Adjust the exposure time until the new print tone is as close as possible to matching your reference tone from step 1. You may want to dry the tests to avoid the dry-down effect.

3. To find the density of the zone negative use the formula Log(New time/Reference time). For example, if your ref. time is 10 seconds and your zone I test negative required 13 seconds for a matching print tone, then the density of the zone I negative is Log(13/10) which is Log(1.3)=0.11394 or about 0.11, good enough for zone I.

4. For the higher density negatives it will be necessary to start with a very low reference time, e.g. 1 second (yes, one second), to avoid excessively long times for the test negative. The longer the exposure time for the test print the more likely you are to encounter reciprocity failure of the printing paper and hence obtain an inaccurate result.

Note: a quick way to test the zone I and zone VIII negs. Make a reference contact print with blank film at 10 seconds to obtain a mid-tone then contact print the zone I neg for 12.6 seconds. If the two print tones match then zone I is 0.10 density above base plus fog. If the zone I print is lighter than the reference tone, the density is more than 0.10. Conversely, if the zone I print tone is darker, then the density is less than 0.10. For the zone VIII neg, make the reference tone from blank film at one second exposure (adjust enlarger height and aperture to obtain mid-tone, it doesn't need to match the other reference tone). Now print the zone III neg for 20 seconds and compare with one second print.

Obviously, this method is not as precise as one might like because it depends on many factors to be kept constant which is not easy or may even be out of your control (such as voltage fluctuations, timer inaccuracies etc.). BUT for people without any other method it will produce workable results. In other words, it will be good enough, if done as accurately as possible, to produce sufficient results for practical zone system photography. Let's face it, I'd rather you did this than do nothing at all to calibrate your system!

Below is a table of exposure times and the corresponding densities based on the reference time of one second. If the reference time is 10 seconds simply subtract 1.00 from all the densities after 10 seconds in the chart e.g. with a ref time of 10 seconds a neg that needs 15 seconds has a density of 0.18. You can easily produce an extended table for your darkroom using a spreadsheet program.