Auto Focus?

[Irie Times]

This may be a dumb question but How do these cameras/lenses auto focus, it is a machine how do they know what in focus is? Pretty amazing if you ask me.

Anyway I was taking some pictures last night and it just struck me as quite impressive that they can do it.

[another view]

Contrast. That's why it can't AF on a smooth wall but can on something like a stone wall or a fence.

[Ron Kruger]

I probably rely on AF less than most, because I do most of my shooting in the woods, and even in the open like to photograph from a low positon to make my subjects bigger than life. AF doesn't distinguish from my subjects and a branch, a weed sticking up between me and my subject, or any other object in the foreground, so I shoot MF most of the time. If you are laying on ground level with your subject, even a flat surface between you and your subject will confuse AF about exactly where to focus.
Yes, it is pretty amazing how they work under ideal conditions and for portriats, but it is a good idea to know their limitations and learn to perfect manual focus. The best way is to go past what looks in focus, come back the other way to the edge and then move back to settle on the focus point. By finding out where each edge of focus is (near and far), you have a better chance of settling upon perfect focus.

[Irie Times]

Contrast that makes sense.

I too usually use Manual mode especially when doing Macro stuff since being off just a little bit makes a huge difference.

thanks for the replies!