Circle of confusion
In the ideal case, a lens intercepts light from a single point on the subject, and focuses it to a perfect pinpoint in the image plane. However if the lens is instead slightly misfocused, this light spreads out into a small blur disk, which by convention is given the rather flowery name circle of confusion (sometimes abbreviated CoC). The largest circle of confusion which is not noticeable, and which thus still appears sharp, is a key factor in calculating of depth of field.
Images which are viewed very critically (i.e. which occupy a very wide portion of the viewer's field of vision) have a more stringent limit on their acceptable circle of confusion diameter. Another factor to consider is the dimensions of the original film (or digital sensor) format: Larger formats require less enlargement to achieve the same viewing conditions. Thus, a larger diameter circle of confusion is permissible.
In creating depth of field tables or calculators, it is typical to use one conventional, or "industry standard" value for the circle of confusion. One common value is to use 1/1,500th of the diagonal of the image format. However this merely corresponds to average viewing conditions, such as holding an 8" x 10" print at arm's length. A more sophisticated depth of field calculator permits the user to select the circle of confusion as an independent value, corresponding to more or less stringent viewing conditions.