Photographic Myth Buster #06

#6. Resolution and Depth of Field

True or False?

The spatial resolution of a camera system (lens, photosensitive array, and supports) determines the field of focus and its depth of field through the diameter of confusion.


Spatial resolution is the ability of a camera system to transfer detail, that is, contrast in tone or color separated by very short distances, from a surface in a scene to an image. It is typically determined by the width of the repeating pairs of the thinnest black and white lines that do not blur together. As the width of these pairs of lines gets smaller, a camera system spreads the lines out into one another blurring any detail. Eventually, the white turns darker and the black turns lighter until the lines are similar grays with no difference in tone or color–no contrast–between them.

The diameter of confusion, which is used to determine the field of focus and its width the depth of field, is defined as the shortest distance in a captured image at which adjacent contrasting tones or colors start to blur together. This is the same situation and distance that is used to define the spatial resolution of a camera system.

Spatial resolution, or simply resolution, is, however, reported as the maximum number of recognizable line pairs or cycles found within a millimeter or within an inch. This is simply the reciprocal of the width (in millimeters or inches) of the recognizable, repeating, black-and-white line pairs. Therefore, the diameter of confusion is simply calculated as the reciprocal of the reported resolution of a camera system.

Diameter of Confusion = 1/Resolution

For instance, the 25 micron (0.025 mm) diameter of confusion often used to calculate the depth of field for emulsions refers to the 40 cycles or line pairs per millimeter resolution typical of emulsions.

The theoretical or limiting resolution for digital cameras is discussed 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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