David Goldfarb , Mar 10, 2003; 03:44 p.m.
There was a thread on this some time back on the Yahoo!Groups Canon FD forum. As I recall, I think the consensus was that you could see the difference, but that it was particularly pronounced if you were to use the 1.4X or 2X extenders. There was one participant who had owned both. I have the 300/4.0L, and will say that it holds up very well with even a the FD Extender 2X-A, which is not true of all FD lenses I've used, and mine isn't even in the greatest condition (purchased from a newspaper that went digital).