#3. The Midtone Reflectance for the Sunny f/16 Rule
True or False?
The Sunny f/16 Rule uses a midtone reflectance that is less than 18%.
As discussed in Photographic Myth Buster #2, the Sunny f/16 Rule requires a light source of 4096 lumen seconds per square meter to give s a midtone exposure with a shutter speed that is one third stop greater than the photosensitivity (film speed) in use.
A photographer can get approximately this luminance by reflecting a portion of the maximum solar illumination of 11,000 lumens per square foot that strikes the earth perpendicularly with a front lit surface, that is, a surface that is held vertically and facing the camera. According to the ANSI Photographic Exposure Guide and the SPSE Handbook of Photographic Science and Engineering, the solar illumination is fairly close to the maximum throughout the middle of the day (between two hours after sunrise and two hours before sunset) during spring, summer, and fall (February through October).
Using a front-lit surface when the sun is behind the photographer and around 60 degrees off the horizon, the solar illumination is reduced two-thirds of a stop (one over two raised to the 2/3 power) by the geometry of the situation.
The Reflection Equation between luminance in candela per square meter and illuminance in lumens per square meter can be used to calculate the reflectance needed to produce the luminance that will be exposed as the midtone of the photosensitive exposure range.
L = (1/π) R E
R = π L / E = [π(4096 cd/m2)] / [(11000 lm/m2) x 22/3 x (3.28 ft/m)2] = 0.1726 or 17%
Using the blue sky on a sunny day rather than the Sunny f/16 Rule to select a midtone exposure means that a photographer is using a 10% midtone reflectance because the luminance of the sky on a clear day at 3,000 candela per square meter is one tenth the luminance of the sun at 30,000 candela per square meter according to the SPSE Handbook. Using the 9570 foot-candles of sunlight and the 1450 foot-candles of skylight that were observed by Jones and Condit in the 1940s, the sky has a midtone reflectance of 15%.
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 Amazon.com (ISBN 978-1-4392-0641-6) where you can Search Inside!™.
Check http://michaelprais.info 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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