I am trying to use an old FD 50mm lens with my Rebel T3i camera but cant get focus. Is there any configuration on the camera I havo to make or it just don't work at all?
Assuming you mean auto-focus, then no, it won't work. AC autofocus for the FD mount lenses only worked with the T80. Assuming you mean manual focus, the focus should work, but it may not be able to focus at certain depths and image quality will likely be greatly compromised. The exact issues you have would depend on the adapter you are using for the mount and how much of an extension tube effect it produces or if it has any corrective optics.