ABSTRACT
Camera lenses are often described by their focal length, which is defined as the distance from the optical center of the lens to the point where the image
is in focus (the surface of the CCD). When we talk about lenses, we usually describe them in terms of how long or short they are. Long and short refer both to the focal length of the lenses and to their relative physical length, since longfocal-length lenses are actually longer than shortfocal-length lenses.
