circle of confusion

Definition: Size of smallest detail that can be distinguished from a viewing distance of 250 mm: usually taken as spot or circle with diameter 0.25mm. * Strictly, circle of confusion is size of spot that cannot be distinguished from true point at a viewing distance of 250 mm. * Any image detail smaller than 0.25mm in a well illuminated subject cannot be distinguished by normal unaided eye at distance of 250 mm: therefore detail finer than this is not required in final print. * This figure is used as basis in lens designs; when calculating depth of field tables and determining screen sizes in printing. * Circle confusion is approximately equal to 1/1000 focal length with e.g. large format or medium format.
Confusingly close
The figures opposite assume that the photograph is viewed from a viewpoint that gives correct perspective. Prints are often examined much closer than 'correct' so sharpness requirements are more critical. In that case, circle confusion may then be taken as 1/2000 focal length or less with 35 mm format. Obviously there is a certain practically determined arbitrariness in the actual figure used.

Related Terms: depth of field

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