In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background. In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus.
Precise focus is possible in only one two-dimensional plane; in that plane, a point object will produce a point image. In any other plane, a point object is defocused, and will produce a blur spot shaped like the aperture, which for the purpose of analysis is usually assumed to be circular. When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as "acceptably sharp". The diameter of the circle increases with distance from the plane of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion, or informally, simply as the circle of confusion. The acceptable circle of confusion is influenced by visual acuity, viewing conditions, and the amount by which the image is enlarged (Ray 2000, 52–53). The increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp.
For 35 mm motion pictures, the image area on the film is roughly 22 mm by 16 mm. The limit of tolerable error was traditionally set at 0.05 mm (0.002 in) diameter, while for 16 mm film, where the size is about half as large, the tolerance is stricter, 0.025 mm (0.001 in). More modern practice for 35 mm productions set the circle of confusion limit at 0.025 mm (0.001 in).
For full-frame 35mm still photography, the circle of confusion is usually chosen to be about 1/30 mm. Because the human eye is capable of resolving a spot with diameter about 1/4 mm at 25 cm distance from the viewing eye, and the 35 mm negative needs about an 8X enlargement to make an 8x10 inch print, it is sometimes argued that the criterion should be about 1/32 mm on the 35mm negative, but 1/30 mm is close enough.
For 6x6 cm format enlarged to 8x8 inches and viewed at 25 cm, the enlargement is 3.4X, hence the circle of confusion criterion is about 1/(3.4 x 4) = 0.07 mm.