You want to fill the frame with a bird in flight, so you rack your long zoom out to the longest focal length. But when you put your eye to the viewfinder, you’re met with an empty field of blue sky or green foliage. Where’s the bird? Rather than swing your lens every which way in an often futile hunt for a moving subject, try zoom tracking. Set your zoom to a wide enough focal length to easily pick up the bird in flight when you put your eye to the viewfinder. As soon as you do, follow focus on it (as with panning, use continuous autofocus), until you’re in synch with its speed and trajectory. Then begin to zoom in and fill the frame. Also like panning, it gets easier with practice.
(From 5 Nature Shooting Skills April 2009 issue)