The Canon Lens Mounts
    As a Canon enthusiast, I have run into a bit of a dilemma. This is a dilemma with which many long time Canon purchasers are familiar. I had, over the course of several years as a hobbyist, put together an extensive system of camera bodies, and lenses based around the Canon FD system. These were bought as need arose, and I soon found myself with a system capable of handling nearly any photographic situation which might reasonably arise. The key to versatility, in any photographic system, is the collection of lenses available to the photographer.
    Canon introduced it's new EF mount for the new line of EOS cameras in 1987, with the unveiling of the EOS 650. What followed was a series of automatic exposure, auto focus, computer controlled electronic cameras. Much of the technology was premiered in the Canon A series of cameras, which had used the old FD series of lenses, but did feature a digital viewfinder, and multi-mode exposure system. One new feature, which had not been on a previous series of Canon cameras, was auto focus. Though the Canon AL-1 had included a focus indicator, in the form of a circle and a pair of arrows, indicating when proper focus was acheived, it had not been linked to the focusing mechanism of the lens.
    Unlike the old FD, FL, and R series of lenses, the new EF type was designed with no mechanical linkages to the camera. Instead each lens has a small digital chip, and motor, and a series of pins connecting to a data bus on the camera's lens mount. The lenses feature a bayonet mount which requies a quarter turn to lock or unlock.

this is not the first time Canon changed it's lens mount.
R, FL, and FD, plus new FD

The two lines coexsisted for a time, but in 1992 canon production of FD lenses ceased.