Photographic Myth Buster #02

#2. Sunny f/16 Rule

True or False?

According to the Sunny f/16 Rule, a photographer should use an f/16 aperture setting and the shutter speed that is closest to the value of her film speed (the photosensitivity) for proper exposure. So, when using an ISO 100 photosensitivity setting, a photographer should use a shutter speed of 125 on traditional cameras with whole-stop settings but a shutter speed of 100 on modern cameras with fractional-stop settings because ISO 100 is an available setting on modern cameras.


Simply, to properly use the Sunny f/16 Rule, a photographer should increase her film speed or ISO Equivalent to the next highest (third-stop) value to get the appropriate shutter speed. So, in the Sunny f/16 Situation, the photographer should always use a shutter speed of 125 when using an ISO 100 photosensitivity. A photographer would only use a shutter speed of 100 under the Sunny f/16 Rule when (in the unlikely situation of) using an ISO 80 photosensitivity.

Here is a partial list of one-third stop photosensitivities and shutter speeds that you find on modern cameras:

30, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500

(If you have a camera with half-stop shutter speed, you have a problem.)

A traditional camera with whole-stop shutter speeds requires a one-third stop jump up from the commercial film speed to get an available shutter speed, but a photographer with a modern camera with fractional-stop shutter speeds must also jump up to the next highest (third-stop) value in calculating the appropriate shutter speed.

Many photographers like to think the Sunny f/16 Rule works because commercial film speeds and ISO Equivalents (photosensitivities) are one third stop below whole-stop film speeds.

The Sunny f/16 Rule is a rule because the common (midtone) Exposure Equation

A2 / t = Ls S / K

makes an unequivocal statement when used in the Sunny f/16 Situation. The Sunny f/16 Situation requires a luminance target (Ls) of 4096 candela per square meter, that is, 12 stops above 1 candela per square meter. Substituting this luminance value along with the f/16 aperture setting (A) and the exposure meter constant K = 12.70 candela seconds per square meter into the common (midtone) Exposure Equation produces an equation for a shutter speed (1/t) that is one third stop greater than the film speed or ISO Equivalent photosensitivity (S).

Shutter speed in stops = Photosensitivity (film speed) in stops + 0.33 stops

All the photographer needs, then, is a luminance target of 4096 candela per square meter. This target is conveniently constructed at about mid-morning on a sunny, summer day. With the sun at our photographer’s back and about 60 degrees off the dawn horizon, a front-lit midtone target–precisely a 17% reflectance target–has the correct luminance. The Sunny f/16 Situation acts as a standard luminance in as much as we can match this situation.

Copyright 2008 Michael G. Prais, Ph.D.

For a readable but in-depth analysis of this concept along with many other concepts associated with photographic exposure, take a look at the book Photographic Exposure Calculations and Camera Operation. This book provides insight into the equations that govern exposure, exposure meters, photosensitive arrays (both solid-state and emulsion) and the Zone System as well as concepts associated with resolution, dynamic range, and depth of field.

The book is available through (ISBN 978-1-4392-0641-6) where you can Search Inside!™.

Check under Photography for the table of contents, an extensive list of the topics and subtopics covered, the preface describing the purpose of the book, and a diagram central to the concepts in the book.

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