A circle of confusion basically describes the smallest photographic image element that preserves details which can still be identified is an
exceedingly contentious and misconstrued variable. This fluctuates with the space from the subject in main focus. A typical human eye can differentiate 5 pairs of lines per millimeter over a distance of 10 inches.
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The eye's resolving power is almost inversely relative to the distance viewed. By doubling the distance, the distinguishable line pairs are cut in half. The converse also becomes true. The normal print viewing distance is typically expressed as somewhat equal to the print's diagonal size. Therefore, a print that is 4x6 is typically viewed at around 7 inches however an 8x10 would most likely be observed at around 13 inches. The acquired image is normally quite a bit smaller, so it must have a much higher resolution.

The field depth is the area whereby a circle of confusion size is smaller than the human eye's resolution (or the medium the images is displayed upon). Circles having a diameter smaller than a circle of confusion therefore will seem in focus.

Two significant uses of the term and concept must be recognized

To calculate a depth of field for a camera, one must know just how big this circle of confusion may be contemplated to be in adequate focus. The acceptable maximum diameter of this type of circle of confusion typically is known as it's maximum allowable circle of confusion, circle of confusion limit of diameter, or circle of confusion criteria, however is frequently incorrectly called simply just circle of confusion.

Out in the real world, lenses never focus every ray perfectly under even under ideal of situations, a lenses circle of confusion becomes a categorization of its optical point. The phrase circle of least confusion typically is used to describe the smallest optical point a lens can achieve, for instance by selecting a best focus location which makes a decent compromise between the changeable effective focal lengths with diverse lens zones because of spherical or supplementary aberrations. The effects of diffraction arising wave optics plus the finite lens aperture may be included in this circle of least confusion, additionally this phrase may be applied to pure ray optics.

Using idealized ray optics, anywhere rays are presuppose to converge to some point when focused perfectly, the form of an
un-focused point from a lens containing a circular aperture becomes a disk of light with a hard edge (sort of a hockey-puck layout as the intensity is charted as a behavior of x and y matches within the focal plane). A more generalized circle of confusion contains soft edges caused by aberrations and diffraction, and may become non-circular because of the aperture's diaphragm character. So the concept of diameter needs careful defining to have any meaning. The overall diameter of a minimum circle which may contain at least 90% of it's optical power is an acceptable definition for a diameter of a circle of confusion. Using the description of the perfect hockey-puck shape, provides an answer of around 5% less than it's actual diameter.
Oct 19, 2011