Editing Circle of confusion (section)

==Two uses==
Two important uses of this term and concept need to be distinguished:

{{ordered list
|1= [[File:Convex lens - perfect.svg|thumb|In a perfect lens {{mvar|L}}, all the rays pass through a focal point {{mvar|F}}. However at other distances from the lens the rays form a circle.]]
For describing the largest blur spot that is indistinguishable from a point. A lens can precisely focus objects at only one distance; objects at other distances are ''[[defocus]]ed''. Defocused object points are imaged as ''blur spots'' rather than points; the greater the distance an object is from the plane of focus, the greater the size of the blur spot. Such a blur spot has the same shape as the lens aperture, but for simplicity, is usually treated as if it were circular. In practice, objects at considerably different distances from the camera can still appear sharp;{{sfn|Ray|2000|p=50}} the range of object distances over which objects appear sharp is the [[depth of field]] (DoF). The common criterion for "acceptable sharpness" in the final image (e.g., print, projection screen, or electronic display) is that the blur spot be indistinguishable from a point. 
{{clear}}

|2= [[File:Convex lens - circle of confusion.svg|thumb|In an imperfect lens {{mvar|L}}, not all rays pass through a focal point. The smallest circle that they pass through {{mvar|C}} is called the circle of least confusion.]]
For describing the blur spot achieved by a lens, at its best focus or more generally. Recognizing that real lenses do not focus all rays perfectly under even the best conditions, the term ''circle of least confusion'' is often used for the smallest blur spot a lens can make,{{sfn|Ray|2002|p= [https://books.google.com/books?id=cuzYl4hx-B8C&pg=PA89 89]}} for example by picking a best focus position that makes a good compromise between the varying effective [[focal length]]s of different lens zones due to spherical or other [[Aberration in optical systems|aberrations]]. The term ''circle of confusion'' is applied more generally, to the size of the out-of-focus spot to which a lens images an object point. [[Diffraction]] effects from wave optics and the finite [[aperture]] of a lens determine the circle of least confusion;<ref>{{cite book
 | title = Recording, modeling and visualization of cultural heritage
 | chapter = Virtual reconstruction of heritage sites: opportunities and challenges created by 3D technologies
 |editor1=Manos Baltsavias |editor2=Armin Gruen |editor3=Luc Van Gool |editor4=Maria Pateraki | author = J.-A. Beraldin
 | publisher = Taylor & Francis
 | year = 2006
 | isbn = 978-0-415-39208-2
 | page = 145
 | chapter-url = https://books.google.com/books?id=ZahHZwpM55YC&pg=PA145
 |display-authors=etal}}</ref> the more general usage of 'circle of confusion' for out-of-focus points can be computed purely in terms of ray (geometric) optics.<ref>
{{cite book
 | title = The technics of the hand camera
 | author = Walter Bulkeley Coventry
 | publisher = Sands & Co.
 | year = 1901
 | page = 9
 | url = https://books.google.com/books?id=zWkOAAAAYAAJ&pg=PA9
 }}</ref>
}}

In idealized ray optics, where rays are assumed to converge to a point when perfectly focused, the shape of a defocus blur spot from a lens with a circular aperture is a hard-edged circle of light. A more general blur spot has soft edges due to diffraction and aberrations,{{sfn|Stokseth|1969|p=1317}}{{sfn|Merklinger|1992|pp=45–46}} and may be non-circular due to the aperture shape. Therefore, the diameter concept needs to be carefully defined in order to be meaningful. Suitable definitions often use the concept of [[encircled energy]], the fraction of the total optical energy of the spot that is within the specified diameter. Values of the fraction (e.g., 80%, 90%) vary with application.