#13. Photographing an 18% Reflectance (Gray) Card
True or False?
The result of metering and photographing an 18% reflectance (gray) card is an image with that card appearing at 18% gray.
The result of image creation is an image with the tones (and colors) produced by the photosensitive process. The photosensitive exposure range of the process has a midtone (midpoint) that depends on the extent of the range.
It is possible to create a photosensitive midtone through development as Adams has or through manipulation in a photo editor, but there is no commonly used emulsion or solid-state photosensitive array with a standard 18% reflective midtone.
Whichever luminance (reflectance) of the scene is selected and exposed to be at the photosensitive midtone appears as that midtone. It doesn’t matter whether subject exposed to be at that midtone is 18% or 80%, photography transforms that subject to the midtone of the photosensitive exposure range.
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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