The Use of Equivalent ASA Numbers

Testing Method 1 is designed to allow you to test your film at a wide range of ASA numbers on the same roll or box of film. In this way, you will determine which ASA is best for your purposes. To do this, you are going to bracket your exposures on each roll according to a predetermined plan. The principle that governs this process is very simple.

Bracketing your exposures is the same as changing the ASA on your meter. Consider this demonstration:

1. Set your light meter to ASA 400 and place meter number 12 opposite the Zone V arrow. If you were testing Tri-X, normally rated ASA 400, and had placed meter number 10 on Zone III, your first exposure would be f/16 at 1/60 of a second (Figure 58).

FIGURE 58 Zone III placement of meter number 10 at ASA 400. Exposure: f/16 at 1/60 of a second.

2. Stop down one-half stop from this starting point without changing your ASA. Your exposure is now 1/60 of a second at f/16-22 (Figure 59).

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FIGURE 59 One-half stop less exposure at ASA 400. Exposure: f/16-22 at 1/60 of a second.

3. If instead of stopping down, you were to reset your ASA to 600 without changing your placement of meter number 10 on Zone III, the result would also be 1/60 of a second at f/16-22 (Figure 60).

These examples demonstrate that after you have established your placement of the Important Shadow Area, stopping down has the same effect as shooting at a higher ASA number. As far as the film is concerned, there is no difference. Of course, this principle also operates in the other direction: Opening up is the same as using a lower ASA number.

In testing Method 1, you will be filling in a column on the Exposure Record labeled "ASA" (column C in Figure 62). The ASA numbers that you enter in this column will be the ASA numbers that are equivalent to the exposures in the adjacent columns. For example, Figure 60 shows that ASA 600 is equivalent to stopping down one-half stop from f/16 at 1/60 of a second at ASA 400. At no point in the test should you change the ASA setting on the dial of your meter. Set your ASA dial to the number recommended by the manufacturer, then stop down or open up as indicated by the exposure plan that is appropriate for your film format. After you have calculated and entered your exposures on the Exposure Record, enter the equivalent ASA numbers in the column to the right.

This process will be reviewed here, but it may be useful for you to work with this concept using your meter dial until it is clear.

FIGURE 60 Zone III placement of meter number 10 at ASA 600 (between ASA 800 and 400). Exposure: 1/60 of a second at f/16-22.
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