Photographic Myth Buster #08

#8. Shutter Speed and f/Number

True or False?

A one-stop increase in the shutter speed and a one-stop increase in the aperture number (the f/number) both decrease the quantity of light reaching a photosensitive array.

True.

As noted earlier in Photographic Myth Buster #1, the (relative) shutter speed is the reciprocal of the exposure time. So, the exposure time is essentially the reciprocal of the shutter speed. The exposure time and the quantity of light reaching a photosensitive array decrease as the photographer increases the shutter speed.

Camera manufacturers use the f/number notation to describe lens apertures because the diameter of the aperture is precisely the focal length f divided by a number, the aperture number A.

Diameter of Aperture = f/A = Focal Length / Aperture Number

Increasing the aperture number decreases the diameter of the aperture and the quantity of light reaching a photosensitive array.

Because both the shutter speed and the aperture number control the quantity of light reaching a photosensitive array through their reciprocals (one over their values), increasing either quantity decreases the quantity of light reaching a photosensitive array and vice versa.

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.

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

 

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