The 200/2, 300/2.8, 400/2.8, 500/4.5 all have a calibration slot that can adjust the focusing element holder forward and backward. To get infinity you have to loosen the fastener on each side slightly, using gravity let the lens and holder module slide as far forwards as possible and retighten. In a few cases I have heard of with the 300 this didn't quite get to infinity and the issue was overcome with using a small circular file to extend the slots a tiny bit. It is also important with all the FD super telephoto lenses to have one of the UV filters installed, it affects the focal path and helps in reaching infinity. Check out EdMika.com for the videos on how to do this with the 400 2.8, the method is the same for all the above listed lenses. -Ed
Thanks for the tip on the UV filter- I wasn't aware of that.
Another post on the net noted infinity focus just with micro adjustment and no callibration - on other sites, you often note "FD 500mm 4.5L, unknown" so I wasn't sure if the process was the same.