Thanks useful information: 
I will add to Tannin's wonderfully explained post, that you have to remember that with some lenses, the autofocus is controlled by the lens. Some lenses are able to focus faster than others. The same as some lenses are 'sharper' (a better focus on the subject and crisper image result) than others. Having a fast autofocus for good results, comes down to the camera, the lens and the photographer. Do not expect an upgrade (to a newer) camera is going to result in better photos. Because if you are having focus issues now, it could well be the lens used or the operator, not the camera body.
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