Critical focus
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In a photograph, the area of critical focus is the portion of the picture that is optically in focus. This does not relate to depth of field which describes apparent sharpness. Changing the aperture will increase the depth of field but the plane of critical focus will not change. Depth of field extends away from the plane of critical sharpness.
The image is only critically in focus along a plane. The formula that describes the relationship between plane of sharpness, lens and film is 1/I+1/O = 1/F
(I is the film to lens distance) (O is the distance from the lens to the plane of critical focus) (F is the focal length of the lens)
See also: depth of field
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