Research- Depth of Field and Focus

Focal length and diaphragm aperture affect the depth of field of a scene — that is, how much the background, mid-ground and foreground will be rendered in "acceptable focus" (only one exact plane of the image is in precise focus) on the film or video target. Depth of field (not to be confused with depth of focus) is determined by the aperture size and the focal distance. A large or deep depth of field is generated with a very small iris aperture and focusing on a point in the distance, whereas a shallow depth of field will be achieved with a large (open) iris aperture and focusing closer to the lens. Depth of field is also governed by the format size. 70 mm film has much more depth of field for the same focal length lens than does 35 mm. 16 mm has even less and most digital video cameras have less depth of field than 16 mm. But if one considers the field of view and angle of view, the smaller the image is, the shorter the focal length should be, as to keep the same field of view. Then, the smaller the image is, the more depth of field is obtained, for the same field of view. Therefore, 70mm has less depth of field than 35mm for a given field of view, 16mm more than 35mm, and video cameras even more depth of field than 16mm. As videographers try to emulate the look of 35 mm film with digital cameras, this is one issue of frustration - excessive depth of field with digital cameras and using additional optical devices to reduce that depth of field.

In Citizen Kane (1941), cinematographer Gregg Toland and director Orson Welles used tighter apertures to create very large depth of field in the scenes, often rendering every detail of the foreground and background of the sets in sharp focus. This practice is known as deep focus. Deep focus became a popular cinematographic device from the 1940s onwards in Hollywood. Today, the trend is for more shallow focus.

To change the plane of focus from one object or character to another within a shot is commonly known as a rack focus.

Source: http://en.wikipedia.org/wiki/Cinematography#Depth_of_field_and_focus