Expressed in millimeters (mm), this refers to how much of the scene can be captured by the lens. It is the distance between the lens' theoretical center to the imaging sensor. If the focal length is longer than 50 mm, it is a telephoto lens. If it is shorter, it is a wide-angle lens. In the case of digital SLR cameras with sensors smaller than full frame, the lens focal length equivalent to the 35mm format, or crop factor, is often stated.