Canon 1D Mark III autofocus
focus is on the player with the ball, but a split second later, it has shifted to player 26 entering the inner AF circle closer to the camera than the other players - lesson 1 - AF will tend to focus on the closest player within the AF circle.
Geelong's Tom Hawkins kicking for goal - initially good focus but inexplicably focus is temporarily lost then regained in this 10fps burst. I have included the full image and a enlarged crop to demonstrate the focus. No player entered the scene, so why was focus lost and reverted to the foreground grass?
focus is regained - presumably camera's AF algorithm detects closest moving object.
2 frames later, focus starting to be lost again - presumably as subject is now outside the inner AF circle.
Lesson 2: keep the subject within the AF circle instead of trying to compose the image.
Lesson 3: this confirms lessons 1 & 2 - keeping subjects within inner AF circle and avoiding closer subjects entering scene ensures AF is maintained - in this case on every frame of this 10fps burst.
AF not yet achieved as camera just moved onto this subject.
Lesson 4: AF is achieved quite rapidly in this conditions and works well to follow the closest moving subject within the inner AF circle.
2 frames later AF is inexplicably lost.
Lesson 5: can't always predict AF will work through a sequence.
see my more descriptive photo story of the game here: sports - Geelong vs Essendon July 2007