Editing Circle of confusion (section)

{{Short description|Blurry region in optics}}
{{redirect|Blur circle|the film|Blur Circle}}
{{For|the closely related topic in microscopy|Point spread function}}
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{{Technical|date=January 2020}}
{{More citations needed|date=January 2020}}
{{Over-quotation|date=May 2021}}
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{{Use American English|date = March 2019}}
[[File:Circles of confusion lens diagram.svg|thumb|Diagram showing circles of confusion for point source too close, in focus, and too far]]

In [[optics]], a '''circle of confusion''' (CoC) is an optical spot caused by a cone of light [[ray (optics)|rays]] from a [[lens (optics)|lens]] not coming to a perfect [[focus (optics)|focus]] when imaging a [[Point source#Light|point source]]. It is also known as '''disk of confusion''', '''circle of indistinctness''', '''blur circle''', or '''blur spot'''.

In photography, the circle of confusion is used to determine the [[depth of field]], the part of an image that is acceptably sharp. A standard value of CoC is often associated with each [[film format|image format]], but the most appropriate value depends on [[visual acuity]], viewing conditions, and the amount of enlargement. Usages in context include ''maximum permissible circle of confusion'',  ''circle of confusion diameter limit'', and the ''circle of confusion criterion''.

Real lenses [[defocus|do not focus]] all rays perfectly, so that even at best focus, a point is imaged as a spot rather than a point. The smallest such spot that a lens can produce is often referred to as the '''circle of least confusion'''.