Kszok and saea501,
Thanks for your responses. I shoot a range of things, probably focussing (no pun intended) on birds / birds in flight, and sports of all kinds. in fact my current project is to capture as many high-class sporting events in my home town of Canberra (we often host international events). So far i have a collection of cricket, baseball, football, tennis, fencing, and field hockey, and I am looking forward to an upcoming car rally through the forests around Canberra. I expect archery will be a challenge!!
After your comments, i looked back at my 400mm shots. I can see that the majority of my "people" shots are quite good, but with smaller subjects such as birds, i don't have as many crisp shots. So that to me suggests "operator-error", and some more learning for me. I note, however, that my success rate with my Sigma 2.8 70-200 APO DG is much higher - consistently sharp and clear.
I will be posting today (hopefully) some of these shots in their respective forums.