As I mentioned in Discovering Your Custom Film Speed, the first, critical step in mastering your medium is to know the starting point for your film, or that point where a film first begins to show noticeable changes due to exposure to light. This is sometimes called the "true ISO" or "true film speed". Interestingly, this true ISO can vary from photographer to photographer. Every serious photographer eventually learns where it is for the films they use, and all your exposure decisions should be based on your awareness of this critical point.

Use your personalized film speed in place of the manufacturers speed when setting the ISO on your camera. With this personalized speed, exposures can be made with confidence that shadow detail will be recorded accurately, and the need for bracketing exposures to ensure a good shot will be reduced.

In the print article, I outlined one, low tech, but fairly accurate way of determining your "true ISO." A more precise method is to make a contact print of the seven-frame negative you made in the earlier test.

The Contact Print Method
To use this method you first make a test strip so that the boarder between each of the test strip bands segment one frame on your film. Process and dry the print. You should now have a piece of paper with several, progressively-darker bands over seven progressively-darker squares. (Note: You should be able to see the sprocket holes of your film along most of it.) Examine the print along the sprocket holes and find the two darkest bands where the film's edge is equally dark in both. Of those two, the band that represents the shorter exposure time is the one you want. Now make a contact print using that exposure time. Process and dry the print completely. Now, using an 18% gray card for comparison, you should be able to determine which frame matches the gray card in density. This frame represents your true ISO.

Using a Densitometer
The most accurate method utilizes a machine called a densitometer to measure the amount of light not shining through the film. If your school's darkroom does not have one then perhaps your local photo lab will read the densities for you. Beginning with the seventh frame, measure each consecutive frame for density and write the number next to the corresponding film speed. The seventh frame is the one you made with the lens cap on and should be free of emulsion. This makes it the perfect starting point because the density you measure comes only from the base plus some fog.

Now, subtract the "base plus fog" number from each density. The result closest to point one (.10) is your own personal film speed or "true ISO."

If there are two film speeds listed next to your density number, then use the lower ISO if the density is greater than point one, and the higher ISO if the density is less than point one. If none of the densities is close to point one, then the test will have to be re-done using a greater range of exposures.

With your personalized speed you can shoot with confidence that shadow detail will be recorded in full, and the need for bracketing exposures to ensure a good shot will be reduced, perhaps eliminated.

For this article, Mr. Richardson used Arista.EDU 400 with Arista 76 developer processed for 9.5 minutes. His densitometer results were: Film base = .26 .02 .03 .05 .07 .08 .10

View Camera Users
For those who have access to a view camera, consider yourself lucky. View camera lenses adjust in 1/3 stop increments eliminating the need to double on your chart. You shoot one normal, three over and three under at 1/3 increments. To get the required blank sheet, view camera users should develop one unexposed sheet of film. That's a total of 8 exposures in stead of seven, but your precision will be greater.