John or anyone else ,this might help understand why AFA Servo mode is not recomended
“The problem with using AF-A is that it’s impossible for the camera to know what your photographic intentions are. Let’s say you are photographing a footrace like the 100-meter dash. Before the race starts, all the runners are in their positions on the starting blocks. The camera senses that all the runners are stationary, so it automatically chooses AF-S. When the runners jump off the line and sprint down the track, the camera will be stuck in AF-S mode when it should be in AF-C mode. You’ll get one sharp photograph of the beginning of the race, and the rest of the photos will be out of focus because the AF system didn’t track the runners in motion.
If the camera starts out in AF-S mode and you need it to use AF-C mode, you have to lift your finger off the shutter-release button after the athletes start moving, then quickly press the shutter-release”
“button again to resume AF. This will cause you to miss some shots while the camera adjusts to the change.”
Excerpt From
The Nikon Autofocus System
Mike Hagen