A nice, simple explanation: "So, what should you do? Well, if you are using lens and aperture combinations that give you a lot of depth of field to work with, focus and recompose can work out fairly well. On the other hand, if you’re working with wide apertures or long lenses, then you’ll need to be more careful. No matter what the situation, your best bet is to use the focus point that is closest to your subject if you can and avoid recomposing altogether."