Canon FD lens
The Canon FD lens mount is a physical standard for connecting a photographic lens to a single-lens reflex camera body. The standard was developed by Canon of Japan and was introduced in March 1971 with the Canon F-1 camera. It was the primary Canon SLR lens mounting system until 1987 when the cameras from the Canon EOS series were first produced using the new EF lens mount. The last camera in the FD system was the Canon T60, from 1990. The Canon FD mount replaced Canon's earlier FL mount (which in turn had replaced the R mount); FD-mount cameras could use FL lenses in stop-down metering mode. There is no known meaning for 'FD', and Canon has never disclosed what, if anything, it stands for.
The 42mm flange focal distance of the Canon FD lens mount is shorter than that of most other lens mounts. Although FD lenses can be mounted on most other types of camera with the appropriate adapter, the lenses cannot focus to infinity unless the adapter contains an optical correction element. Canon FD lenses can however be mounted on Canon rangefinder cameras or other Leica screw mount cameras using the Canon lens mount adapter 'B', but rangefinder-coupled focusing is lost. Canon FD lenses can also be mounted without optical correction on the Micro Four Thirds system, which has a flange focal distance of only 20mm. The 2x Crop factor of the Micro Four Thirds system means that the Field Of View is halved.