I was shooting some type of Heron, (type not important) and spot focus, no expansion. Center point. I was focusing on the head of the bird. In the background at about 300 yards were houses and trees. Nothing but water between the bird and the houses and trees background. I had the focus point on the head and the camera kept trying to focus on the background.

It would shift back and forth and when it would finally lock on the head I would shoot. Was this due to the fact that the camera focusing actually encompasses more area than just the spot focusing square in the viewfinder? So maybe just outside of the spot square it was also picking up on the contrasty background? And the head of the bird was very narrow compared to the wide area in the background at 300 yards.

I hope I'm making sense. I'm kind of answering my own questions as I write this. Just never had this problem before. But then I've never shot something so isolated in the foreground with so much contrast in the background.

If I understand your question correctly, than I think you answered your own question. Indeed, as far as I am aware, the "spot" is larger than what appears in the viewfinder. Easy to slip off of the intended target when hand-held. At least that's my read on the situation.

Happy New Year!

makes sure you set your focusing so that it does not easily switch in the custom functions. one of the things is to set focus seek when focus is impossible (or something like that) to disable, and that helps the back and forth a lot.

kevindar wrote:
makes sure you set your focusing so that it does not easily switch in the custom functions. one of the things is to set focus seek when focus is impossible (or something like that) to disable, and that helps the back and forth a lot.

I've been afraid to set that particular function for fear the it will give up focusing when it is focused on the wrong thing. I'll have to read about that one some more as I'm not sure exactly how it works. I understand that it will stop the lens from hunting but when it stops what happens?

And a Happy New Year to yous guys also.

If you are going to be doing any serious wildlife or BIF photography, you need to learn and become comfortable with using your focus ring for rapidly putting your subject in the general vicinity of correct focus as needed. this is the reason behind having a usm lens with full time manual focus.
if you subject fills your viewfinder, or most of your view finder, focusing is easier. of course most of the time that is not the case.