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Definition: focal length

Focal length is the optical distance in millimeters between a camera lens and the sensor or film. It determines the height and width of the scene being captured, known as the "field of view."

A prime lens has one focal length and thus a fixed field of view. In order to change it, the photographer has to move closer or farther away from the subject. A zoom lens has a range from wide angle to telephoto.

Focal length is one of the two primary measurements of a camera lens. The other is the "aperture," which is the lens opening (see f-stop).

The Field of View
These photos were taken with an 18-70mm zoom lens on a digital SLR (see DSLR). At 18mm (top), the field of view is the greatest (wide angle). At 70mm (bottom), the scene is the narrowest (telephoto). This lens is equivalent to a 27-105mm lens on a 35mm SLR camera (see crop factor).