This is the lens I decided to adapt to fit a Canon EOS camera which has an EF lens mount. The EF mount has a 44 mm and the FD is 42 mm flange to film or sensor distance. So some metal would have to be removed from the back end of the FD if one expected it to fit on the EF. There was no particular reason for choosing this lens except that it would allow low light level use either on a EOS film or a Digital Canon SLR camera. I happen to have the original Canon Rebel Digital and it has served me well over the last few years. With a maximum aperture of f/2 this lens is still a specialty lens when it comes to maximum aperture for the focal length. And when used on the Rebel the 135 mm lens would would appear to operate as a 200 mm lens at f/2. Amazing. It is a reality that reasonably priced adapters to allow mounting an FD lens to an EF mounts are available. These are essentially a small telextender or Barlow lens. They introduce a small extension tube within which there is a negative lens element and this allows infinity focus of the FD lens although the ÒpriceÓ to pay for this is a 1.25X increase in focal length and a 1 stop loss in speed. PLUS a small to large loss in image quality due to the introduction of the optical element between the lens and the film or sensor plane.