Many photographers rely completely on exposure data from their camera’s built-in light meter.  In today’s world of matrix metering on sophisticated digital cameras, this produces good results under many lighting conditions lighting conditions.  However, most early vintage cameras do not have light meters, and even a highly automatic light meter is not automatically right.  Experienced photographers frequently employ an extraordinarily useful rule of thumb known as the “Sunny f/16 Rule.”  This simple rule states:

“On a sunny day, for correct exposure, set the camera’s aperture at f/16 and the  shutter speed to the reciprocal of the ISO film speed.”

In other words, when shooting Tri-X, ISO 400, on a sunny day with no haze or cloud and a front-lighted subject, a perfect exposure will be obtained by setting the aperture to f/16 and the shutter speed at 1/400 sec.  For practical purposes, the nearest calibrated shutter speed of 1/500 will do fine.  For Panatomic-X at ISO 100 on the same sunny day, optimum exposure will be obtained with f/16 at 1/100 sec.  For other film types, just plug in the ISO film speed as the reciprocal of the shutter speed.

This rule is important for three reasons:

1. It allows one to work without a light meter.  There are situations where experienced professional photographers do not use their exposure meters, but depend on this rule.
2. Since the amount of incident light from the sun is quite constant (at least in the latitudes that most of us occupy) the rule can be used as a way of checking the accuracy of one’s light meters.
3. Light meters measure either incident or reflected light (see Sekonic’s site for a discussion of incident vs. reflective metering).  Incident light readings, measuring the amount of light falling on the subject, are most accurate, but must be taken with the meter close to the subject.  Depending on the circumstances (see below), this may be impractical.  Reflected light measurements are most common (all on-camera meters measure reflected light), but the amount of light reflected toward the meter depends on the lightness of the subject.  Since most meters are calibrated to the amount of light reflected from a 14-18% neutral gray surface, it is necessary to adjust the reading for darker or lighter subjects.    The Sunny f/16 Rule, representing really the ultimate measure of incident daylight falling on the subject,  bypasses these problems, and is surprisingly accurate.

Incident Metering is a Bad Idea. Use the Sunny f/16 Rule

This rule can be tabulated for different film speeds as follows:

The Sunny f/16 rule can be extended to other lighting conditions, producing a table similar to that provided by many manufacturers on their film packaging:

An extremely detailed tabulation of empirical exposure settings for a variety of light conditions is available on Karen Nakamura’s Photoethnography web site under the “Exposure” tab.

References:

Author Unknown.  “Guide to Film Photography:  The Sunny 16 Rule.”  http://guidetofilmphotography.com/sunny-16-exposure.html.

Folds, Ben.  “Sunny 16 Rule.”  http://en.wikipedia.org/wiki/Sunny_16_rule.

Glatzer, Charles.   “Sunny f/16 Rule.”  http://www.photomigrations.com/articles/0403200.htm.

Nakamura, Karen.  “The Science of Photography:  Exposure.”   Photoethnography web site.  http://johnlind.tripod.com/science/scienceexposure.html.

Ratty Brian.  “Understanding the Sunny f/16 Rule.”  http://www.photomigrations.com/articles/0403200.htm.

Sekonic Inc.  “Incident vs. Reflective.”   http://www.sekonic.com/classroom/classroom_2.asp.