In optics, the focal length of a lens is the distance between the lens-air interface and the focal point of the lens, or the point at which all the light rays entering the lens converge to form a sharp image of the object from which the light rays have been reflected.
In photography, the focal length is defined as the distance from the center of the lens to the sharp image of an object (i.e., focal point), when the lens is focused at infinity. (See Photography.)