Quote Originally Posted by Stephen Benskin View Post
From the ANSI - Exposure Guide

Attachment 44202 Attachment 44203
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8.3 Equivalent Incident-Light Values. The incident-light values (Iv) listed in Tables 1 to 10 are based on daylight (sun plus sky) measurements made with the photocell perpendicular to the direction of the sun on a clear day. The effectiveness of the light in exposing film in a camera varies with the sun-subject-camera angle. The light is most effective with the sun directly behind the camera, with an angle of 0°. In this situation, the equivalent incident-light value (I'v) is the same as the incident-light value (Iv).

Exposure adjustments for the factors in the checklist will be applied by decreasing or increasing incident-light value and identifying the adjusted number as the equivalent incident-light value.

9.1 The Key f-Number Rule. According to the well-known f/16 rule, the correct exposure is produced with a shutter setting of I/ISO (ASA) film speed (ISO arithmetic film speed) at an aperture setting of f/16 on a clear day with the subject facing the sun. This rule should produce the correct exposure in all situations in which an incident-light value (Iv) of 10-2/3 appears in the latitude-month-hour tables (given in Tables 1 through 10), providing that:

(1) The sun-subject-camera angle is approximately 55° to 80°, producing an equivalent incident-light value (I) of 10, and

(2) No adjustment is required for any other exposure factors.
The f/16 rule can be modified to include other incident-light values (Iv) and compensations for any exposure factors that apply by relating key f-numbers to the equivalent incident-light values (Iv).
Key f-numbers and equivalent incident-light values appear in the first two columns of the nomograph in Figure 20.

The procedure used to find the key f-number is as follows:

(1) Find the incident-light value (Iv) for the latitude, month, and hour in the appropriate table from Tables 1 through 10; for example, 10-2/3.
(2) Total any exposure adjustments needed, from the checklist of exposure factors in Table 11; for example, +2/3 stop for lighting direction.
(3) Subtract the second number from the first when an exposure increase is required, or add the second number to the first when an exposure decrease is required; for example, 10-2/3 — 2/3 = 10. Identify this number as the equivalent incident-light value (Iv).
(4) Select the key f-number from the first column corresponding to the equivalent incident-light value (Iv) in the second column of the daylight nomograph in Figure 20; for example, f/16.
(5) Expose the film at the key f-number and a shutter setting of 1 / ISO (ASA) film speed, or any comparable combination; for example, f/16 and 1/60 second for an ISO (ASA) 64 film speed.

9.2 Exposure Nomograph. The procedure when using the exposure nomograph is as follows:
(1) Find the incident-light value (Iv) for the lati¬tude, month, and hour in the appropriate table from Tables 1 through 10; for example, 10-2/3.
(2) Total any exposure adjustments needed from the checklist of exposure factors in Table 11.
(3) Subtract the second number from the first when an exposure increase is required, or add the second number to the first when an exposure decrease is required; for example, 10-2/3 — 2/3 = 10. Identify this number as the equivalent incident-light value (I'v).
(4) Locate the equivalent incident-light value (I'v) in the second column of the daylight nomograph in Figure 20 and align a straightedge from I'v to the ISO (ASA) film speed of the film being used (S) in the sixth column. Note where the straightedge crosses the exposure value (Ev) scale in the fourth column; for example, for an I'v of 10 and an ISO (ASA) 64 film speed, Ev = 14.
(5) Rotate the straightedge about the Ev to select any combination of f-number and exposure time in the second and fourth column; for example, f/8 and 1/250 second."
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