Photographic Myth Buster #27

#27. The ISO Equivalent Setting for Raw Data

True or False?

The ISO Equivalent (photosensitivity) setting on a camera is extraneous when processing “raw” exposure data.


The ISO Equivalent photosensitivity points to the midtone exposure of an eight-bit (eight-stop) section of a larger range of sensor data. Each sensor might have 12, 14 or 16 bits (or stops) of data for each color. When selecting the data associated with an ISO Equivalent photosensitivity, data greater than 255 are counted with the data for 255 and data smaller than the eight-bit zero (the less significant digits) are counted with the data for zero.

When downloading “raw” data from a camera, each of the bits of data for each sensor is downloaded as 16-bit numbers with 12, 14 or 16 bits of useful information. There is no conversion of the data to an eight-stop range or into eight-bit values.

The ISO Equivalent (photosensitivity) setting only affects the selection and presentation of “cooked” sensor data. It has no affect on the measurements as each pixel collects data over its full 12, 14 or 16 stop (bit) range irrespective of the ISO setting.

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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