#33. Midtone Reflectance of the Zone System
True or False?
The nine-stop (nine-zone) photosensitive exposure range used by Ansel Adams for his Zone System has a midtone reflectance of 4.4%.
Using the equation that describes the relationship between the midtone reflectance of a range of reflectances and the width of a range of exposures in stops that was discussed in the explanation in Photographic Myth Buster #32,
log2(1.00/Rm) = Width of the Range/2
it is simple to show that a nine-stop range has a 4.4% midtone reflectance.
1.00/Rm = 2Width of the Range/2 = 29/2 = 24.5
Rm = 1/24.5= 0.0442 or 4.4%
Ansel Adams shifts the midtone reflectance of his prints during processing from 4.4% to 18% producing the wider range of shadows at the expense of a narrower range of highlights for which he is well known and well regarded.
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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