Photographic Myth Buster #26

#26. The Midtone Exposure and the Photosensitivity (Film Speed)

True or False?

The midtone exposure of any emulsion or solid-state media determines its photosensitivity (film speed).


While many photographers believe that the photosensitivity (film speed) S is determined by the measured value of the speed point exposure Hsp and a constant (the reference exposure Ho),

S = Ho / Hsp

the explanations in Photographic Myth Buster #24 and in Photographic Myth Buster #25 show that the reference exposure Ho should not be arbitrarily assigned a value, should be recognized as dependent on the width of the photosensitive exposure range (through the midtone shift M) of the photosensitive array under consideration, and, as such, produces a photosensitivity (film speed) that is determined solely by the midtone exposure Hm.

S = Ho / Hsp = (qo K / M) / Hsp = qo K / M Hsp = qo K / Hm

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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