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focal length - technical definition
The height and width of the scene captured by a camera lens, which is known as the "field of view." In the past, lenses with fixed focal lengths were commonly used, which means every picture was captured with the same field of view. Today, most lenses are "zoom lenses" with a variable range of focal lengths and a variable field of view. See f-stop.
Wide Angle to Telephoto
These two pictures were taken at both ends of a Nikon 18-70 mm zoom lens on a digital SLR. At 18 mm (top), the field of view is the widest. At 70 mm (bottom), the scene is the narrowest. The 18-70 mm on the Nikon camera is equivalent to a 27-105 mm lens on a 35 mm analog SLR camera (see crop factor).
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