To reduce the length of a telephoto lens, it is necessary to increase the mutual power of the convex-concave groupings. Fluorites low index of refraction makes it possible to achieve significant reduction in lens length while maintaining high image quality. Although the extraordinary optical properties of fluorite were discovered in the 19th century and lens designers have long desired to use it, naturally formed pieces of fluorite large enough for use in lens production are extremely difficult to find. Deciding to solve this problem, Canon took up the challenge of developing synthetic crystals, bringing practical fluorite production technology on-line by the late 1960s.canonglossaryeu
An optical reduction of a film from one gauge to another, such as 35mm to 16mm.
See blowdown. Making a copy of a film original on smaller format raw stock by optical printing; for example, printing a 35 mm original onto 16 mm stock.