I own a Nikon D7000 which of course has 39 focus points. I was going through some articles on internet on focusing techniques. One article I came across was say shooting people (portraits) I should use AF-S and Single focus point (central only to be specific) I should point it on eyes preferably and press the shutter button half way once the camera locks it focus, press down the AE-L/AF-L button to keep the focus locked and then re compose my shot. e.g. rule of thirds or something else. I am confused over how come I can not compose my shot first and then chose any of the other 39 focus points which points on my subject. If the central point is more advanced from others, then how come when central focus point is not even on my subject, will help in taking a sharp picture that has sharpest details of my subject.