# Photographic Myth Buster #22

#22. The Reference Exposure

True or False?

The reference exposure Ho, which appears as the numerator in the definition of the photosensitivity (film speed) in terms of the speed point exposure Hsp, S = Ho / Hsp, is the speed point exposure for ISO 1, that is, for a photosensitivity of one.

True.

When the photosensitivity (film speed) S is one (S = 1) in the equation that most photographers take to define it,

S = Ho / Hsp

we have with a quick rearrangement Hsp = Ho or Ho = Hsp. In words, the reference exposure Ho (the numerator in this definition of S) is the speed point exposure Hsp when the photosensitivity S = 1.

While the numerator (the upper expression in a division or ratio) is not identified as the reference exposure in any of the sensitometric (film speed or photosensitivity) standards, each of the emulsion and solid-state standards define particular values for the numerator that can collectively be called reference exposures.

The reference exposure and the speed point exposure both carry the units of exposure which cancel when divided by the speed point exposure to give a photosensitivity S that does not carry any physical units.

The photosensitivity S = 1 is such a unique numerical value among photosensitivities that the speed point exposure associated with this value of the photosensitivity could easily be called a reference exposure.

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 http://michaelprais.info 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.