Photographic Myth Buster #41

#41. Photosensitivity (Film Speed) and Midtone Exposure

True or False?

The photosensitivity (film speed) of an emulsion or a solid-state photosensitive array points to the midtone exposure of the photosensitive range of the array.


This is a novel concept, and it bears repeating. Most photographers would be willing to bet that the photosensitivity (film speed) is determined from the speed point exposure Hsp through the equation S = Ho / Hsp. In practice, the photosensitivity often is because the reference exposure Ho is assigned an arbitrary value within the sensitometric (film speed or photosensitivity) standards. This occurs because these standards do not recognize that the standards for exposure meters place the requirement on the value for the reference exposure Ho that it describes the width of the photosensitive exposure range around the midtone of the range that is described by the photosensitivity (film speed).

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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