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