Focal Length

The focal length of a digital camera lens is the distance between the center of the lens and the image sensor when an in-focus image is formed. The focal length of a digital camera lens is displayed on the barrel of the lens along with the measurement of the largest aperture and the maker.
The focal length of a lens establishes the field of view of the camera. The shorter the focal length is, the larger the field of view.
Camera lenses are categorized into normal, telephoto, and wide angle, according to focal length. Thus a 200 mm equivalent telephoto lens gives a 4 x magnification over the 50 mm equivalent lens.