| FD 55mm ƒ/1.2 AL | |
| Maker: | Canon |
| Flength: | 55 mm |
| Aperture: | ƒ/1.2 – ƒ/16 |
| Groups: | 6 |
| Elements: | 8 |
| Diaphragm: | 8 |
| Max-Mag: | 0.11 |
| Filter: | 58 mm |
| Start: | March 1971 |
| Stop: | 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]