The Canon FD lens mount is a physical standard for connecting a photographic lens to a 35mm 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 served as the Canon SLR interchangeable lens mounting system until the 1987 introduction of the Canon EOS series cameras, which use the newer EF lens mount. The FD mount lingered through the release of the 1990 Canon T60, the last camera introduced in the FD system, and the end of the Canon New F-1 product cycle in 1992. The FD mount was based upon and replaced Canon's earlier FL mount (which in turn had replaced the R mount); FD-mount cameras can use FL lenses in stop-down metering mode. Though never officially explained by Canon, others have attempted to assign a meaning to the "FD" designation. One such attempt states that the "FD" notation stands for "Focal-plane shutter with Dual linkage for diaphragm control"; in actuality, there were two linkages and two signals: the automatic aperture lever, aperture signal lever, full aperture signal pin, and automatic exposure lock pin. This is twice the previous lens series, which used the "FL" designation, said to mean "Focal-plane shutter, Linked mount." Over the 21-plus years of production, Canon introduced 134 different FD lenses ranging from 7.5mm through 1,200mm in seventeen different fixed focal lengths and nineteen different zoom ranges, one of the most, if not the most, extensive manual focus lens lines ever produced. The Canon FD system enjoyed huge popularity in the 1970s and 1980s, when it established and grew a market share with professional photographers as well as having equipped over a million consumer users. Indeed, sales of the Canon AE-1 camera alone exceeded one million. Canon obsoleted the FD mount by its decision to create the all-electronic EF mount. Thus, the FD mount system, with limited provision for auto-focus, is now commercially obsolete, and Canon FD cameras and lenses are available for low prices on the second-hand market. This makes the system very attractive to 35mm film photographers who demand the highest optical quality, while not needing auto focus capability. FD lenses can be used on many mirrorless interchangeable-lens cameras with a suitable adapter. They are a popular alternative to modern lenses among some users, though they lack autofocus. Background The FD lens mount is a breech-lock mount, which is