The flange focal distance of the FD mount is smaller than most other lens mounts, meaning that it is impossible to mount a FD lens onto most other cameras with an adapter and maintain the same focusing distance. Canon made an adapter containing corrective optics allowing its larger, more expensive FD telephoto lenses to be used on EOS bodies. This adapter functioned as a mild tele-converter, and could not be used on normal and wide-angle lenses. Canon produced this adapter to ease the initial cost of conversion for those with expensive collections of long FD telephoto lenses.