Wikipedia
Deep focus is a photographic and cinematographic technique using 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 use of the hyperfocal distance of the camera lens.
Deep focus is achieved with large amounts of light and small aperture. It is also possible to achieve the illusion of deep focus with optical tricks (split focus diopter) or by compositing two pictures together. It is the aperture of a camera lens that determines the depth of field. Wide angle lenses also make a larger portion of the image appear sharp. The aperture of a camera determines how much light enters through the lens, so achieving deep focus requires a bright scene or long exposure. Aperture is measured in f-stops ( T-stops on lenses for motion picture cameras are f-stops adjusted for the lenses' light transmission, and cannot be used directly for depth of focus determination) with a higher value indicating a smaller aperture.
The opposite of deep focus is shallow focus, in which only one plane of the image is in focus.
Orson Welles and Gregg Toland were most responsible for popularizing deep focus through its use in Welles's film Citizen Kane.
Usage examples of "deep focus".
I slip into deep focus and the knife becomes part of me, not a thing in my hand any more but an extension, part of my body.
There was only the Viewer, slumped forever in his sour seat, the bald shells of his eyes boiling in pictures, a biblical flood of them, all saturated tones and deep focus, not one life-size, and the hands applauding, always applauding, palms abraded to an open fretwork of gristle and bone, the ruined teeth fixed in a yellowy smile that will not diminish, that will not fade, he's happy, he's being entertained.