Photographic Myth Buster #39

#39. Exposure Range of ISO Equivalents

True or False?

An ISO Equivalent setting on a “digital” camera points to the midtone exposure of eight-stop photosensitive exposure range of the integral solid-state photosensitive array.


The typical (“cooked”) output of a “digital” camera is the eight-bit colors of sensor data that is associated with an ISO Equivalent photosensitivity. Each of the three colors (RGB) at each photosensitive location of the sensor is described by eight bits of data. The eight bits of data identifies 256 possible color values over an eight-stop range.

(The colors are associated here with a “photosensitive location” rather than a “pixel” because the typical solid-state photosensitive array uses four pixels in a RG/GB mosaic pattern to determine the three colors associated with the four pixels in that location.)

The explanation in Photographic Myth Buster #26 shows that any photosensitivity (including any ISO Equivalent photosensitivity) points to the midtone exposure of an exposure range. An ISO Equivalent photosensitivity should, therefore, point to the midtone exposure of its eight-stop photosensitive exposure range.

Since the actual (“raw”) photosensitive exposure range of a digital camera is much larger than eight stops (perhaps 12, 14, or 16 stops), the ISO Equivalent exposure ranges are a series of overlapping eight-stop ranges centered on a series of ISO Equivalent photosensitivities.

The diagrams that support the calculation of the standard width of the photosensitive exposure range for various types of photosensitive arrays can be found in the book Photographic Exposure Calculations and Camera Operation.

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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