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 composite 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 mise en scène. Aperture is measured in F-stops (T-stops on lenses for motion picture cameras are f/stops adjusted for the lens' light transmission. Do not use T-stops 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.

In the cinema Orson Welles and his cinematographer Gregg Toland were most responsible for popularizing deep focus. Their film Citizen Kane (1941) is a veritable textbook of possible uses of the technique.