Originally posted by stl09 Single point on auto focus...will try it out without the CP before contact Sigma
I'm far from an expert here so someone correct me if I am wrong, But so far as I understand it, auto focus works on contrast. Imagine a black box in the middle of a white page. When it is out of focus, the edges of the black box are spread out and blurred. When it is in focus, the edges are sharp and defined. Your auto focus is measuring that edge where it transitions from black to white. When the mixed blurred grey area becomes smallest, its in focus, or something along those lines anyway. If you shoot at a white wall, there is no contrast so your camera has a hard time auto focusing. If it does auto focus it probably found some imperfection in the wall where there was a little contrast and locked on it. Focusing on Mount Rushmore, which is basically rock of similar color, there are less areas of significant contrast so it is harder to auto focus. I'm just guessing but if the camera is trying several point instead of one it might have an easier time finding a good point of contrast to lock onto.
This is of course just one rough theory of one of the many possibilities. One way to test would be to print a page that is half black and half white, and see if you can focus on it. Maybe even better a page with several sharp wide lines. Of course with a long lens like that you will probably have to mount it on a wall a ways away or something.
I just googled focus test chart thinking I could find a page you could print and stumbled on this (a much better explanation than mine).
Nikon D70 Focus Chart