# Photographic Myth Buster #25

#25. Determining the Photosensitivity (Film Speed)

True or False?

The photosensitivity (film speed) of a photosensitive array is most accurately determined from the minimum usable exposure and the maximum usable exposure of the array.

True.

The answer in Photographic Myth Buster #23 showed that the midtone exposure is determined by the sum of the maximum usable exposure and the minimum usable exposure divided by two.

Midtone Exposure in Stops = log2(Hm) = [ log2(Hmax) + log2(Hmin) ]/2

The answer to question #24 showed that the photosensitivity (film speed) of a photosensitive array is determined by the midtone exposure of the photosensitive array.

S = qo K / Hm

Putting those two statements together, we have that the photosensitivity (film speed) of a photosensitive array (an emulsion or a solid-state array) is determined by the sum of the maximum usable exposure and the minimum usable exposure of the photosensitive array.

log2(S) = log2(qo K) – log2(Hm)

= log2(qo K) – [ log2(Hmax) + log2(Hmin) ]/2

Given those two exposures for an array, a photographer can calculate the photosensitivity of that array.

This is a novel statement. 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 reference exposure Ho is simply assigned a convenient value within the sensitometric (film speed or photosensitivity) standards without recognition that the standards for exposure meters place a requirement on the value for the reference exposure Ho.

While the sensitometric standards ignore this requirement, the construction of exposure meters makes use of this requirement in a very practical way as is shown in the answer to question #28.

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

Check https://michaelprais.me 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.