By: Thomas Harrop
There are myriad ways to test film speed. Many require a densitometer and because most photographers don’t have access to one, I am offering a simple test that you can do in your back yard. If you are careful with the test you can establish a film speed that works well for your equipment and processing style.
Here is the test procedure:
1) Create a scene to photograph which contains the following items: Something black, something white and a gray card. In addition I usually include a person as a flesh tone reference.
2) Photograph the scene using the following exposure sequence:
Frame 1) Metered at the film’s rated ISO speed.
Frame 2) Open up two stops from the first exposure.
Frame 3) Open up one stop from the first exposure.
Frame 4) Open up one half stop from the first exposure.
Frame 5) Close down one half stop from frame one.
Frame 6) Close down one stop from frame one.
Process the film as you normally process film then make a proof using the minimum time for maximum black we established in the last test. The resulting contact sheet will show you the actual exposure values created in your film.
To determine your personal film speed simply select the frame that looks best to you and determine where it falls in the exposure series. If frame one looks best you should rate your film at the ISO speed published by the film manufacturer. Film speeds double and half just like apertures so if the frame you judge to be best was exposed at one stop less exposure than the frame one of the test (frame 5) you should rate your film at twice the manufacturers speed. If you are testing film with an ISO of 100, this means you should set your meter at 200. On the other hand, if your best frame was exposed at two stops more open than frame 1, your new personal film speed would be 1/4 the film’s rated ISO. So, if your film was rated at 100, you would now set your meter at 25.
Keep in mind that these new film speed ratings are not ISO speeds. There is a very precise method used to determine ISO and any speed not calculated according the that standard should be called an exposure index or E.I. Therefore, when noting your new film speed you should get into the habit of calling it E.I. 25 or E.I 200 unless it is the actual rated speed.
You should think of these tests as a starting point. Most of the process of learning black-and-white photography is the elimination of variables. It is very difficult to solve any problem when you have dozens of aspects changing. By standardizing your print time and film speed you can begin to concentrate on other aspects of image making.
In the future article I will examine some exposure methods, including a simplified introduction to zone system exposure control. Keep in mind that many, if not most, photographers find that their work gets worse for a while once they begin to approach exposure and tone control seriously. The testing process can be challenging but when you have worked through the problems you will find that you can reach a level of nearly total control over your images. Whatever the short term set backs, that is a goal that is worth working toward.