#28. Exposure Meters Point to Midtone
True or False?
Modern exposure meters point to the midtone exposure of the photosensitive exposure range of the emulsion or solid-state photosensitive array in use.
True, but why do they and what does it mean to point to midtone?
You are wrong if you think that exposure meters point to midtone because they are calibrated to an 18% reflectance or to any other midtone reflectance. The explanation in Photographic Myth Buster #12 discusses how exposure meters as measuring devices are calibrated over a range of illuminances (inputs) with a standard light source held at various distances and over a range of voltages (outputs) with a standard voltage source (and precision variable resistance).
Exposure meters point to midtone–particularly, a midtone photosensitive exposure–because they are calculation devices (as well as measuring devices) that use the (modtone) Exposure Equation
A2 / t = Ls S / K
to get an aperture number A and a shutter speed 1/t from a luminance Ls representative of the scene. Since this common version of the exposure meter works by giving the photographer the camera settings needed to match the representative luminance from the scene to the midtone exposure of the photosensitive range, it is more precisely the Exposure Equation used by the exposure meter that points to midtone.
How does this common version of the Exposure Equation point to the midtone exposure? If you have been following the explanations in Photographic Myth Buster #24 and in Photographic Myth Buster #25, you should recognize that there is a requirement–an equation, Ho M = qo K–implicit in the standards for exposure meters that unequivocally connects the photosensitivity S and the midtone exposure Hm of any photosensitive array.
S = qo K / Hm
S / K = qo / Hm
It is through the photosensitivity S (or through S/K) in the Exposure Equation that the Exposure Equation points to the midtone exposure Hm.
It is, therefore, not the exposure meter per se that points to the midtone, but it is the photosensitivity S of the photosensitive array in a photographer’s camera that points to the midtone exposure that is implicit in the calculation of the camera settings (the aperture number and the shutter speed). The exposure meter points to midtone only after the photosensitivity is set. Like Babe Ruth pointing to left field before the pitch, the photographer points to his or her midtone exposure by picking the photosensitivity of his or her photosensitive array well before handling an exposure meter.
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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