Photographic Myth Buster #01

#01. Shutter Speed

True or False?

The shutter speed for an exposure time setting of 1/125 seconds is 125.


Since any speed is a distance d over a time t, a speed is proportional to the reciprocal (one over) the time it takes to travel a given distance. The shutter speed is the distance the shutter travels divided by the time spent traveling d/t. In this case, that time is the exposure time. Since the shutter on a camera always travels the same distance, the reciprocal of the exposure time 1/t, always being proportional to the shutter speed, is a fair indicator of shutter speed. So, the shutter speed for an exposure time of 1/125 seconds is proportional to its reciprocal 125.

You can be imprecise and say that this shutter speed is simply 125 because it is when you ignore the inconvenient physical units of distance the shutter traveled per second.

You can be precise and say that the relative shutter speed is simply 125 (without any physical units) because the relative shutter speed can be defined as a shutter speed relative to the shutter speed for an exposure time of one second. In the ratio of a shutter speed to the shutter speed for an exposure time of one second, the distance the shutter traveled in both speeds cancels leaving a simple number.

Relative shutter speed = (d/t) / (d/to) = to/t where to is an exposure time of one second

You cannot be accurate and say, as many photographers do, that the shutter speed is 1/125 seconds because a speed is never equivalent to a time (although it can be equivalent to the reciprocal of a time, that is, 125).

(Note that 1/125 second and 125 are the representative exposure time and shutter speed. The actual values are 1/128 and 128, respectively, where 128 is 27 or 7 stops.)

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.

Should you have any comments or questions about this web site, please contact me. Thanks.


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