<P> The focal length of an optical system is a measure of how strongly the system converges or diverges light . For an optical system in air, it is the distance over which initially collimated (parallel) rays are brought to a focus . A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them to a focus in a shorter distance . </P> <P> In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view . On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection . </P> <P> For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens . For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot . For a diverging lens (for example a concave lens), the focal length is negative, and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens . </P>

The focal length f of a camera lens is related