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. In deep focus the foreground, middle-ground and background are all in focus.
Deep focus is normally achieved by choosing a small aperture. The aperture of a camera determines how much light enters through the lens, so achieving deep focus requires a bright scene or long exposure. Wide angle lenses also make a larger portion of the image appear sharp.
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.
Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one. The actual size of the aperture will depend on the focal length of the lens.
The opposite of deep focus is shallow focus, in which the plane of the image that is in focus is very shallow.
When deep focus is used, filmmakers often combine it with deep space (also called deep staging). Deep space is a part of mise-en-scène, placing significant actors and props in different planes of the picture. Directors and cinematographers often use deep space without using deep focus, being either an artistic choice or because they do not have resources to create a deep focus look, or both.
Directors may use deep focus in only some scenes or even just some shots. Other auteurs choose to use it consistently throughout the movie, either as a stylistic choice or because they believe it represents reality better. Filmmakers such as Akira Kurosawa, Stanley Kubrick, Kenji Mizoguchi, Orson Welles, Masahiro Shinoda, Akio Jissoji, Terry Gilliam, Jean Renoir, Jacques Tati, James Wong Howe, and Gregg Toland all used deep focus as part of their signature style. The 14-mm lens has become informally known as "The Gilliam" among filmmakers because of Terry Gilliam's frequent use of it at least since Brazil.
Deep focus and different formats
The choice of shooting format affects how easy it would be to achieve a deep focus look. This is because the size of the sensor or film gauge dictates what particular lens focal length would be used in order to achieve a desired viewing angle. Smaller sensors or film gauges will require an overall range of shorter focal lengths to achieve any desired viewing angle than larger sensors or film gauges. Because depth of field is a characteristic of lens focal length (in addition to aperture and focus distance setting), it is easier to achieve a deep-focus look with a smaller imaging sensor or film gauge. For example: a 40mm lens will give a 30-degree horizontal angle of view in the Super35 format. To achieve the same viewing angle with a 1/2" 16:9 sensor, you would need a 13mm lens. A 13mm lens inherently has much more depth-of-field than a 40mm lens. To achieve the same depth of field with a 40mm lens would require a very small aperture, which in turn would require far more light, and therefore time and expense.