Canon's FD mount came as the next logical step after the FL and R SLR mounts. All three of these used breech-lock systems, although Canon later abandoned this with their New FD (FDn) lenses. All FD lenses can be used on all FD bodies, though, and FL lenses can be used on most FD bodies as well. One big advantage of the FD system was open-aperture metering, where the photographer could focus and meter without manually stopping down the lens' aperture. Another big advantage of the FD system was its capacity for automatic exposure. Cameras like the Canon AE-1 and A-1 are capable of advanced automatic exposure with all FD lenses. Over its two-decade production run, Canon introduced 134 different lenses for the FD system. These range from 7.5mm fisheyes to 1200mm ultra-telephotos. One of the most popular models in the FD lineup was the Canon AE-1 Program, which sold well over one million units during the 1980s. Canon FD lenses, especially the oldest S.S.C models) are regaining popularity in part due to their similarity to Canon K35 cinema lenses. The rise of mirrorless cameras has also given FD lenses new life outside of 35mm photography.