I have a bit of a weird problem.
I am trying to photograph a facetted gemstone and for some reason it will not autofocus onto the stone.
I am using a Canon 400D with a 100mm 2.8L series lens, using spot focus, and shooting Raw in either AV or P mode. I can get a great shot with manual focus so I am not too close, however, even when I manually focus on the stone and revert to auto focus, it will simply blow the focus right out. Have tried various Iso's, f settings, shutter speeds, etc.
Is it because of the clarity of the stone not enabling focus, or the reflections off the facet faces, or what.
Got me a little bushed I must say.