[wrogers]

I only manually focus (as big bill does) when branches or grass are in the way... however my Sigma 150-500 OS does nearly always lock on and very quickly...
I have only had to switch to MF with tiny objects that even a 800mm + 2x tc would struggle with (I only tried this to test the lens)
the other problem is with my dog running, but she is the 3rd fastest breed (smooth collie)
and MF on a running dog at over 30Mph is pretty impossible!!!

unfortunately the 75-300 isn't the best lens at AF and the 400D ain't much better...
however some of us can't afford a 1Dmk3 and 600mm F/4!!!
try the 55-250 IS... at £200 it is a reasonable lens that does work very well for £200... It seems more like a £300-£400 lens!!