Focal lengths are usually specified in millimetres (mm), but older lenses marked in centimetres (cm) and inches are still to be found. The angle of view depends on the ratio between the focal length and the film size. Due to the popularity of the 35 mm standard, lenses are often described in terms of their "35 mm equivalent" fields of view. This is the difference between a normal lens (e.g. 50 mm), wide-angle lens (e.g. 24 mm), and telephoto lens (e.g. 500 mm). This is particularly common with digital cameras, which generally use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to generate equivalent images.