Well I took my newly acquired used :-) EF 100-300 F5.6L lens for testing with the Canon EOS 10D at the london zoo. It's the oldest Canon L zoom (perhaps) made in 1987, uses a prehistoric Arc Form Drive (AFD) motor that reminds you of a coffee grinder when it tries to autofocus. The lens is slow, push pull type, hunts a lot, slow in autofocussing, and shows zoom creep when pointed upwards or downwards. On the positive side, it's very sharp wide open, has nice colour (bit warm though, but i like it) and contrast, lightweight, and (surprise surprise!!!) focusses most accurately of all my lenses on the 10D (of course when it focusses at all) :-). At 480 mm effective focal length on the 10D at the long end this needs 1/500 shutter speed for handheld photos. However, I could manage sharp photos handheld down to 1/90 secs too !!! but I found I'm safe upto 1/350 . Verdict: I am keeping the lens but this might not be an ideal wildlife lens. If you are not interested in technical discussion just enjoy the photos and let me know if you like them :-)