#12. Exposure Meters and 18%
True or False?
Exposure meters are calibrated to 18% reflectance by the ISO/ANSI standards for exposure meters.
Exposure meters as measuring devices are calibrated over a range of illuminances (inputs) produced by a standard light source held at various distances from the measuring device and over a range of voltages (outputs) produced by a standard voltage source and precision variable resistance.
In general, instruments are calibrated over a range of input or output values using two separated points in or at the ends of that range. The calibration produces a set of output values that are proportional to (or linearly related to) the input values.
Offering a single input value, an 18% reflectance (with or without an associated output value), falls short of what is required for calibration.
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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