Focus

Sharp focus on a moving subject requires the following:

1. A subject large enough relative to the focus sensor that a focus sensor can track it independent of the background.  The large the subject is in the viewfinder, the better the focus sensors can track the whole subject or a particular part of the subject.
2. Decent contrast between subject and background
3. A shutter speed fast enough to stop undesired motion blur (what you need depends upon a lot of things such as subject speed, focal length, subject distance, angle of motion relative to camera, etc...).  For reference, I shoot soccer players with a 200-400 at 1/800 or higher.
4. The appropriate AF settings for good tracking on your subject.
5. Continuous focus after acquiring focus on the right part of the subject.
6. Good, stable camera tracking technique on the desired focus sensor.

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John
Gallery: http://jfriend.smugmug.com