Keeping the focus on the main action is sometimes harder than you think. With all the other players moving through your shot, sometimes the camera thinks they are the action and changes focus to them. Singling out the play, and keeping the focus on it is the secret to great shots.
To allow your camera to ‘track’ the play and change the focus as the play moves around, you must set the camera to a continuous focus mode. Canon calls it AI Servo (Nikon calls it AF-C) and you have the camera ‘acquire’ the focus by picking up the action you want in the center focusing spot. Then as the play moves around the frame, the focus can track the play.
This works great until another player gets between you and the focus point. As you may know, autofocus locks onto the object closest to the camera in the focus area. So as a player moves between you and the play, the focus changes to the new player and spoils your shots until you reacquire the original play.
On many digital SLR’s there is a function that allows you to change the way the shutter release and the autofocus lock buttons work. Normally you push the shutter button and it turns on the autofocus and fires the shutter. By changing the autofocus to work when pushing the autofocus lock button, you separate each function to its own button. This allows you to have the shutter button half pressed and ready to fire any time, while toggling the focus with your thumb.
So now you follow the play with your thumb on the focus lock button. As another player moves through the play, release the button so the focus stops. When the player moves away push the button again to reacquire the focus on the play. All this time taking photos with the normal shutter button.
This way of shooting takes a bit of practice to master, but I don’t know of anyone who tries it, who goes back to the old way…At least for shooting sports.