In photography, the circle of confusion (CoC) 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 image format, but the most apprpriate value depends on visual acuity, viewing conditions and the amount of enlargement. Properly, this is the maximum permissible circle of confusion diameter limit, or the circle of confusion criterion, but is simply called circle of confusion.Real lenses do not focus all rays perfectly, and so, 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.nikongrossary
The Circle of Confusion characterizes the degree of acceptable focus. The smaller the circle of confusion is, the higher the resulting image sharpness and narrower the depth of field. Traditionally, CoC s of 0.001 for 35mm formats and 0.0005 for 16mm formats are used. With modern, higher contrast, sharper lenses and finer film grains, smaller CoC s may be desired since a smaller resulting spot on the film is reproducible.
Since all lenses contain a certain amount of spherical aberration and astigmatism, they cannot perfectly converge rays from a subject point to form a true image point (i.e., an infinitely small dot with zero area). In other words, images are formed from a composite of dots (not points) having a certain area, or size. Since the image becomes less sharp as the size of these dots increases, the dots are called circles of confusion. Thus, one way of indicating the quality of a lens is by the smallest dot it can form, or its minimum circle of confusion. The maximum allowable dot size in an image is called the permissible circle of confusion.
Tiny disc of light. Images formed by a lens are made up of these discs. The smaller these are, the sharper the image.