Description:The shapes of the surfaces of a lens which exactly focuses a point object to a point image are hyperbolas. Parallel rays incident on an 18 inch long spherical lens converge at the focal point, but have lots of spherical aberration, as seen in the photograph at the left. A hyperbolic lens is virtually free from spherical aberration, as seen in the photograph at the right. Chromatic aberration is still present, as can be seen by blocking off part of one of the extreme rays. Number of slits and their spacing can be changed by choice of slit baffle and distance of baffle from source.