Canon FD Lens Mount - Background

Background

The FD lens mount is a breech-lock mount, which is a variation of the common triple-flanged bayonet attachment. The advantage of the breech-lock over the bayonet is that neither the contact surfaces between the body and lens, nor the signalling mechanisms, rotate against each other when the lens is mounted. This prevents any mechanical wear, which could conceivably reduce the very precise lens-to-film distance or introduce communication errors between lens and body.

Canon's first iteration of the breech-lock, extended forward from the earlier R- and FL-series lenses, utilized a rotating mounting ring at the rear of the lens; the lens body itself did not rotate for mounting. Its minor disadvantage was a somewhat slower lens change than a bayonet. Second-generation FD lenses, first marketed in 1981 as New FD, are mounted like bayonet-mount lenses in that the photographer twists the entire lens body to mount and dismount, though the actual mating surfaces still remain fixed. This retained the advantages of the breech-lock mount while adding the convenience of a bayonet. Canon later chose a bayonet-style mount for its EOS system's EF lenses, where there is no precision mechanical coupling.

Like its FL predecessor, the FD mount system allowed automatic diaphragm function, but in addition, a new signal pin supported full-aperture metering. A second signal pin for the "auto" setting of the aperture dial, plus a linkage to allow the camera to set the diaphragm opening, enabled integral auto-exposure. The first camera to utilize this was the 1971 Canon F-1, when equipped with the Servo EE Finder. Later, the Canon EF of 1973 had automatic exposure built-in, as did the very popular Canon A-series cameras (save the AT-1) beginning in 1976.

The FD mount has no support for either electrical or mechanical lens-body communication required for autofocus, which was a primary reason for its retirement. (The three AC series lenses, extended from the FD lens mount, described below, are an exception). While Canon could have adapted its mount to support auto-focus, as did other manufacturers, instead the company chose to make a clean break with the past and design a completely new interface with support for electrical signaling.