gregory t. davis

determining your reciprocity adjustments

In the previous article, we determined our personal film speed and our standard development times. In this article, we will determine our exposure adjustments for reciprocity failure. Charts are available for these, but only cover some films and are based on someone else's experience. If long exposure times are common in your work, creating your own chart will be of a great benefit.

Making the exposures

While it is not necessary, it does help if you have six extra darkslides. These can be picked up at bargain prices from eBay, used photo stores, and camera swap meets. To prepare the darkslides for the test, you will need to drill a one inch hole in each darkslide like the figures below.

Number each darkslide for easier reference during the test. Set up your gray card like the film speed test and insert a film holder. Set your shutter to make a Zone 1 exposure using a 1/4 second shutter speed. This serves as your control density. Insert the darkslide you labeled as number one. Every negative you make in this test will use a Zone 1 exposure at 1/4 second using the #1 darkslide.

After making this exposure, replace the #1 darkslide with the #2 and make a Zone 1 exposure at one second. Every negative in this test will use the #2 darkslide to make a Zone 1 exposure at the meter reading without reciprocity adjustment.

For the remaining darkslides, increase the exposure one second, so for this negative, it will be 2 seconds, 3 seconds, and 4 seconds. Then replace the film holder and repeat step one.

For the second negative, meter for and make a 2 second exposure on darkslide #2. With the remaining darkslides make increasing exposures of 3, 4, 5, and 6 seconds. Follow this pattern for all the times you want to test for, always using a 1/4 second exposure in #1 space for reference, and your metered time in the #2 space, then space the remaining exposures to cover an increasing amount for longer metered times. For example, for your 30 second metered time, expose the #2 darkslide for 30 seconds, then increase to 45, 60, 75, and 90 seconds. or somewhere encompassing the time you feel may be correct.

Determining the correct exposure times

Once you have all the negatives you want to make, develop them using your standard development time. You can read the negative densities on a densitometer if you choose. You are looking for the exposure that reads closest to the 1/4 second control exposure. In my test, using Efke pl100, my one second metered exposure was correct, so no reciprocity adjustment is needed, but my two second exposure was off, so I need to expose for three seconds on a two second reading.

If you don't have a densitometer, you can interpret your results by printing the negatives using your standard printing time as explained in the article "Film Speed and Development Time Test by Visual Inspection". Print the negatives and look for the circle with the same shade of grey as the 1/4 second circle. It should be the first shade that separates from the paper's maximum black.

If you test for a good range of exposures, such as 1, 2, 3, 5, 10, 15, 25, 45, and 60 seconds, you can graph the results and extrapolate the exposures inbetween. To do this, get a sheet of graph paper and on the bottom edge list from one to 60 seconds, or what ever time you used as your maximum test exposure. On the left side, list from one to 120. If you can go higher than 120 on your piece of paper, list up to 180.

Plot the exposures that most closely matched your Zone 1 exposures like below.

Using the chart you make, you can test a few exposures spanning a large range of meter readings, then extrapolate the remaining exposures. By doing one or two tests based on your chart, you can verify your findings.

Using just a few sheets of film, a few spare darkslides, and an afternoon, you can can create your own personalized reciprocity chart that is more accurate to your working methods.