Deep focus, depth of focus, depth of field, sharp focus, focus

Deep focus is a photographic and cinematographic technique incorporating a large depth-of-field. Depth-of-field is the front-to-back range of focus in an image — that is, how much of it appears sharp and clear. Consequently, in deep focus the foreground, middle-ground and background are all in focus. This can be achieved through knowledgeable application of the hyperfocal distance of the camera lens being used.
The opposite of deep focus is shallow focus, in which only one plane of the image is in focus.
In the cinema, Orson Welles and his cinematographer Gregg Toland were the two individuals most responsible for popularizing deep focus. Their film Citizen Kane (1941) is a veritable textbook of possible uses of the technique.
Depth of focus vs depth of field
While the phrase depth of focus was historically used, and is sometimes still used, to mean depth of field, in modern times it is more often reserved for the image-side depth. Depth of field is a measurement of depth of acceptable sharpness in the object space, or subject space.

Depth of focus, however, is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus. It can be viewed as the flip side of depth of field, occurring on the opposite side of the lens.
Where depth of field often can be measured in macroscopic units such as meters and feet, depth of focus is typically measured in microscopic units such as fractions of a millimeter or thousandths of an inch.
The same factors that determine depth of field also determine depth of focus, but these factors can have different effects than they have in depth of field. Both depth of field and depth of focus increase with smaller apertures. For distant subjects (beyond macro range), depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases.
Calculation

A rough formula often used to quickly calculate depth of focus is the product of the focal length times the f-stop divided by 1000; the formula makes most sense in the case of normal lens (as opposed to wide-angle or telephoto), where the focal length is a representation of the format size. The precise formula for depth of focus is two times the f-number times the circle of confusion times the quantity of one plus the magnification factor. However, the magnification factor depends on the focal length and format size and exact focus the lens is set to, which can be difficult to calculate. Therefore, the first formula is often used as a guideline, as it is much easier to calculate. It relies on the historical convention of circle of confusion limit equal to focal length divided by 1000, which is deprecated in modern photographic teachings, in favor of format size (for example, along the diagonal) divided by 1000 or 1500.