The optical design of a binocular or spotting scope. A binocular can be
either a porro prism (whose objective lenses are off-set and spaced
further apart than the eyepieces) or a roof prism (whose objective
lenses are in line with the eyepieces). A spotting scope can be either a
porro prism or roof prism refractor or a catadioptric (a combination of
lenses, mirrors, and prisms).