#4. "RE: Is this the D5100 'backfocus' problem?" In response to In response to 3 Thu 01-Aug-13 08:40 PM by ericbowles
3D tracking could be a bit of a problem, but most likely it does not make any difference. 3D tracking is for situations when a subject is moving toward the camera - for example, your girlfriend walking, running, or biking toward you. It tracks the subject as it moves toward you.
Single point AF is quite precise - it does not guess what the intended subject is and where you should focus. That being said, the dynamic focus modes are also good if you have a moving subject that is hard to track. Use the smallest number of focus points that allows you to keep the indicator on the subject. If you are struggling to keep the indicator on the subject, 9 point Dynamic is a good option. If you struggle to get the subject in the frame - you can use all the AF points. In all cases, only the focus point you intend to use is highlighted.