I am a little confused by all the auto focus options offered on my D7100. I am trying to set the camera for capturing birds and military aircraft in flight, usually against a sky background, using my Sigma 50-500 OS lens at 500mm. I would like to swing the subject into the viewfinder, half-press the shutter release, and have the camera find the subject, focus on it, and hold focus even as the moving subject shifts around in the viewfinder while panning. To do this, I am using AF-C focus mode with 51 focus points. Is this the best way to accomplish my objective, or should I be using 3D Tracking or Auto-Area AF?