Photographic Myth Buster #11

#11. Photosensitivity (Film Speed) and 18%

True or False?

ISO/ANSI sensitometric (film speed) standards use 18% reflectance to define the photosensitivity (film speed) for emulsions.


The ISO/ANSI sensitometric (film speed) standards make no mention of an 18% reflectance or of any midtone reflectance. They simply define the location of the speed point exposure Hsp relative to the minimum usable exposure Hmin (in terms of the minimum usable density Dmin) and the reference exposure Ho.

S = Ho / Hsp

The location of the speed point exposure relative to the minimum usable exposure offers a fractional-stop safety factor in the case of negative emulsions.

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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