From Wikipedia:
[edit] Using FD lenses on other mounts
The 42mm
flange focal distance of the FD mount is shorter than that of most other
lens mounts. This means that it is impossible to mount an FD lens on most other types of cameras using an adapter, unless the adapter contains corrective optics or the ability to focus at infinity is sacrificed. (A notable exception is that Canon FD lenses can 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.)
Following the introduction of the EOS camera line, Canon briefly marketed an adapter which enabled FD telephoto lenses to be used on EOS bodies. The adapter contained high-quality corrective optics and functioned as a mild
tele-converter; it could not be used on normal and wide-angle lenses. The adapter was produced in limited numbers, with the intent of easing the initial cost of conversion for professional users with expensive FD telephoto lenses. These adapters are now scarce and highly valued, selling for around $1000 on the second-hand market. (The original 'new' price was $250). Other inexpensive aftermarket FD to EOS adapters are available and can be used at the cost of greatly reduced image quality, particularly at large apertures.
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Mechanically skilled photographers and technicians have successfully retrofitted FD lenses with alternative mounts, including the EOS mount.
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