#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.
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