#23. The Midtone Exposure
True or False?
The midtone exposure of an exposure range is determined by the minimum usable exposure and the maximum usable exposure of the range.
The midtone exposure of an exposure range is simply the midpoint of an exposure range. To determine the midpoint of any range (or any ordered set of numbers), we need to know the minimum and maximum values in that range of numbers. In the case of an exposure range, the minimum usable exposure and the maximum usable exposure describe the ends of an exposure range.
In looking at an exposure range with its wide variety of values for exposure, photographers typically examine the logarithm of the values for exposure. These values are related to the optical densities of developed photosensitive emulsions through their characteristic curves. Examining the binary logarithm, log2(), of the exposures allows the photographer to calculate the extent of an exposure range in stops as a difference
Extent of an Exposure Range in Stops = log2(Hmax) – log2(Hmin)
and to calculate the midtone exposure in stops as a sum.
Midtone Exposure in Stops = log2(Hm) = [ log2(Hmax) + log2(Hmin) ]/2
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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