<human>:  Where does the focal length of a lens come from?
<bot>: Focal length is the distance from the lens to the point where the image is focused. It is measured in millimeters. The focal length of a lens is determined by the type of lens and the type of camera. For example, a 50mm lens is a standard lens that is used in most cameras. It is a 50mm lens because it is 50mm long.