Photographic Myth Buster #34

#34. The Midtone Reflectance of Jones and Condit

True or False?

In the 1940s, Jones and Condit found that the “typical” exposure range for the scenes that they examined had a contrast ratio (a maximum to minimum exposure ratio) of 160:1 and an 18% midtone reflectance.


The “typical” 160:1 contrast ratio that Jones and Condit found for their mostly sunny, front-lit scenes in Rochester, New Your, was the average of the logarithms of the contrast ratios of those scenes. The contrast ratios ran from 1:27 to 1:720 with a large fraction of bright scenes.

The midtone exposure of this scenic exposure range is determined from the number of stops in the range, log2(160) = 7.3 stops. Following the calculations in the answer to question #33, the midtone reflectance for this range is 8.0%–not 18%.

log2(1.00/Rm) = Width of the Range/2

1.00/Rm = 2Width of the Range/2 = 27.3/2 = 12.55

Rm = 1/12.55 = 0.07966 or 8.0%

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 (ISBN 978-1-4392-0641-6) where you can Search Inside!™.

Check 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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