FD 55mm ƒ/1.2 AL

Maker	Canon
Technical data
Focal length	55 mm
Aperture (max/min)	ƒ/1.2 – ƒ/16
Close focus distance	0.6 m (24 in)
Max. magnification	0.11
Diaphragm blades	8
Construction	8 elements in 6 groups
Physical
Max. length	55 mm (2.2 in)
Diameter	75.8 mm (2.98 in)
Weight	605 g (21.3 oz)
Filter diameter	58 mm
History
Introduction	March 1971
Discontinuation	1980

The Canon FD 55mm ƒ/1.2 AL is a camera lens made by Canon, first introduced alongside the Canon F-1 single-lens reflex camera in March 1971. It was the first lens for any 35mm SLR system to incorporate an aspherical element.^[1] The lens was manufactured until 1980.

The FD 55mm ƒ/1.2 AL uses a variation of the double-Gauss lens design, in which the positive meniscus element of the front Gauss pair is aspherical. In total, the lens has 8 elements in six groups: a front element, two Gauss pairs, and three additional rear elements.

In March 1975, Canon introduced a version of the lens, known as the FD 55mm ƒ/1.2 S.S.C. Aspherical.^[2] Although, this version was the first to formally signify that it is multicoated, the original version without S.S.C. engraving has used this type of multicoating as well.^[3]

Like a number of other contemporary lenses, the Canon FD 55mm ƒ/1.2 AL incorporated thoriated glass,^[4] a kind of optical glass that is doped with thorium dioxide. The inclusion of this compound makes the glass radioactive.

Over time, thorium decay causes F-centers to form in the glass, resulting in an amber discoloration. The discoloration can be repaired by exposure to a source of ultraviolet radiation, such as direct sunlight.^[5]