The default option, in most cameras, is Automatic AF, or also called AI SERVO, when you half press the shot button, you will see the camera trying to focus on the near subject and if you move or the subject move, the camera will keep changing the focus point to always keep the nearest subject on focus. Now this mode is really good for subject that keep moving, but really how many times I need to shot moving subject? Not many at least for me. The problem with this mode is that you can’t compose your shot, if I want to focus my subject that is behind some other objects, I can’t use this mode, the camera will try to focus on the foreground object instead of focusing on my subject behind it. Another composition that might not work is when you want your subject on one side of the frame, in order to achieve this composition, you need to change the point of focus, but this slow down the process a little bit and depending on the camera it might not be that easy to change, you can also achieve this composition using the tracking mode but I will not talk about it since for me is just too cumbersome.