Thank you Jack that was exactly the sort of informative response I was hoping for
A cheap adapter can be bought from places like Ebay enabling you to use FD glass
on an EOS mount without further modification.
Yeah but... There are two types. Since the flange-focal length of the FD mount is very close to that of the EOS mount, the adapter needs to either add space like an extension tube or add optics like a teleconverter. So one is an extension tube and will prevent infinity focus while the other has optics in it and acts like a tele-converter at about 1.3x base focal -- so the 35 becomes a 45. And then you have the issue of the quality of those inexpensive optics. Canon themselves made an optical one that was supposed to be pretty decent, but regardless you have the focal length change.