Photographic Myth Buster #15

#15. Incident-light Exposure Meters

True or False?

Incident-light exposure meters assume that scenes have a 9% midtone reflectance, and they match that scenic midtone to the photosensitive midtone of the photosensitive array in use.


All exposure meters must calculate a luminance representative of the scene at hand so that they can use it in the common (midtone) Exposure Equation

A2 / t = Ls S / K

to calculate the camera settings (the aperture number A and the shutter speed 1/t) that expose that luminance Ls representative of the scene as the midtone of the photosensitive exposure range in use. This equation uses the photosensitivity S of the photosensitive array chosen by the photographer and the exposure meter constant K.

Incident-light exposure meters use the Reflection Equation to calculate a representative luminance for the scene from the measured ambient illuminance.

Ls = ( 1/π ) Rs Ea

When Rs is a reflectance representative of the scene that produces the luminance Ls under ambient illumination Ea, these two equations are often combined along with the definition

Cs = π K / Rs

to calculate the camera settings for incident-light exposure meters from the measured ambient illumination Ea.

A2 / t = Ea S / Cs

Nevertheless, these meters implicitly require a reflectance Rs representative of the assumed scene in order to calculate the representative luminance and the camera settings. The value of this representative reflectance is calculated from the values of two exposure meter constants K and Cs published in the standards for exposure meters.

Rs = π K / Cs

The values of K are allowed to be in the range 10.6 to 13.4 candela seconds per square meter. The values of Cs are allowed to be in the range 320 to 520 lumen seconds per square meter. These values allow the representative reflectances to be in the range 6.40% to 13.2% (multiplying by π to convert from candela to lumens), so the standard representative reflectance used by inicident-light exposure meters is 8.77%. The luminance created by this reflectance is matched by the common (midtone) Exposure Equation to the photosensitive midtone of the photosensitive array in use.

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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