quality:corruption
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. 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 early video cameras, as well as most modern consumer level video cameras, even more depth of field than 16mm.

In Citizen Kane (1941), cinematographer Gregg Toland and director Orson Welles used tighter apertures to create 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 onward 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.

Early in the transition to digital cinematography, the inability of digital video cameras to easily achieve shallow depth of field, due to their small image sensors, was initially an issue of frustration for film makers trying to emulate the look of 35mm film. Optical adapters were devised which accomplished this by mounting a larger format lens which projected its image, at the size of the larger format, on a ground glass screen preserving the depth of field. The adapter and lens then mounted on the small format video camera which in turn focused on the ground glass screen.

Digital SLR still cameras have sensor sizes similar to that of the 35mm film frame, and thus are able to produce images with similar depth of field. The advent of video functions in these cameras sparked a revolution in digital cinematography, with more and more film makers adopting still cameras for the purpose because of the film-like qualities of their images. More recently, more and more dedicated video cameras are being equipped with larger sensors capable of 35mm film-like depth of field.