PIXIPORT-Telephoto Lenses an article by Landscape Photographer Monte Nagler

By definition, a telephoto lens is one whose focal length is greater than that of a normal lens. The longer the focal length of the lens, the greater the degree of magnification. In fact, the magnification is mathematically related to focal length. For example, a 100mm lens produces an image exactly twice that of a normal lens. A 400mm lens gives you an image size eight times your normal lens.