I have a good collection of Canon IS/AF lenses. I own one Sigma lens 50-500mm. I can use it only on a solid tripod (Bogen with a ball head). The AF is fine, but I miss the IS that the other lenses have to damp the minor vibrations. I shoot outdoors always and mainly birdsso a tripod mounted lens is only occasionally useful. I get great shots of the moon with a doubler combined with that lens. The maxfocussing point is somewhere past the moon's orbit! I am going to make the plunge for a Canon fixed 600mm IS/AF and see if it can be hand-held. I'll be back with a post on how it works. BTW I use a Canon 20D with a 6MB microdrive. Love all 497 RAW images I get.
ps: will a Canon doubler lens work with the 600mm + Canon 20D? By work, I mean will it still autofocus? Thanks.