#16. Reflected-light Exposure Meters
True or False?
Reflected-light exposure meters assume nothing about the exposure range of a scene, and they simply match the calculated representative midtone of the scene to the photosensitive midtone of the photosensitive array in use.
All exposure meters must calculate a representative luminance for 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.
Reflected-light exposure meters use the Illumination Equation to calculate a representative luminance for the scene from the measured illuminances in the scene.
Es = ( π/4 ) t qs Ls / A2
This equation can be rearranged to calculate the luminance Ls from the measured illuminances in the scene Es and other known quantities.
Ls = ( 4/π ) A2 Es / t qs
Because this equation only requires parameters about the lens (aperture number A and transmittance T) and the camera-subject geometry (qs), and because this equation uses illuminances measured directly from the scene, reflected-light exposure meters can calculate a representative luminance for the scene without assumptions about the scene.
Reflected-light exposure meters can integrate (average) the light reflected from all the surfaces in the scene to create a representative illuminance for the scene Es. The Illumination Equation is then used to convert that representative illuminance into a representative luminance Ls that is used in the common (midtone) Exposure Equation
A2 / t = Ls S / K
to get the camera settings (the aperture number A and the shutter speed 1/t) that match (expose) that representative luminance to the midtone exposure of the photosensitive exposure range 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 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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