Depth of focus DOF is a lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. 
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. Since the measurement indicates the tolerance of the film's displacement within the camera, depth of focus is sometimes referred to as "lens-to-film tolerance."