page_title: Canon FD lens mount
section_title: Background
hierarchical_section_title: Canon FD lens mount / Background
caption_attribution_description: English: Close up shot of an FD lens, cropped. Taken with 2nd Canon AE-1.
context_page_description: 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; 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.
context_section_description: 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 FD 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. 
  There are three different versions of breech ring FD lenses: 
  The first version had a chrome plated front barrel (nicknamed "chrome nose") and a green "o" for auto exposure.
  The second version had a black front barrel and still had a green "o" for auto exposure.
  The third version had a black front barrel, and a green "A" for auto exposure. These different versions are detailed below under variants.
  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 enabling the convenience of a bayonet. The letters SC or SSC, to indicate the lens coating, were no longer put on the lenses. Canon documents stated that All new FD lenses except for the new FD 50mm F1.8 had SSC coatings. 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 degree of 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.
  Thus, starting with the first FD lenses produced in late 1970, all FD lenses had the capability of supporting full-aperture metering and multiple Automatic Exposure (AE) modes using both shutter-preferred and aperture-preferred modes. Even Programmed AE was possible with no modifications to the lens mount, though at the time of its introduction Canon did not have an AE camera body in the FD line. This was a design triumph for Canon that no other camera or lens maker was able to equal in 1970. Every other camera manufacturer had to make one or more alterations to its lens mount to enable full aperture metering, and later AE and or Programmed AE operation.
  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, the company instead chose to make a clean break with the past and design a completely new interface with support for electrical signaling and control.
caption: Canon FD, f/1.4, 50mm, on AE-1 Program.
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