Canon 1D Mark III autofocus take 3 - success!
Here is a good example of how the AF algorithm works. This sequence is not a very good example of the sport or photo aesthetics.
Because of this I have used full crops but resized to 600x400 which makes it hard to see the shift of focus that occurs but I am sure you can detect it even on these sizes - if not, just take my word for it as I describe what's happening.
Using the EF 135mm f/2.0L with 1.4x EX II extender at ISO 1600 to give 1/800th sec at f/4.0 (with tele-extender).
I used the shutter button to set focus whilst attempting to ensure the subject was on the center AF point to acquire focus to give this first image:
First image above shows the pack of players in front of the goals in reasonable sharpness and certainly these 4 players appear to be the sharpest objects in the image, so AF acquired.
This second image above is 4 shots later in the sequence and still shows AF is maintained on the pack of 4 players despite a foreground umpire running into the scene and entering the AF points zone. If C. Fn III-4 had been left at the default 1 setting, there is a good chance AF would have rapidly shifted to him.
Above, 2 shots further on and even though the umpire has ran through the AF points and the pack of 4 players are falling to the ground, focus is still on the pack of 4 players - so to me - this is success - I have got the settings working nicely for this style of photography at last!
Above, AF fails and defers to the crowd in the background, but I guess this has to be accepted as 3 of the 4 players have now fallen away from the dominant central AF point, although you could argue that they are still within the AF point zone and so the algorithm should have still kept AF on them. But I'm not complaining here, it has done a good job and I'm happy at last.
Same settings as sequence 1 but ISO 800, 1/1000th sec at wide open (f/2.8 with the tele-extender).
Above, first image has AF nicely acquired on the central player in navy, but as this sequence unfolds, it is the player running in from the right which is important to the image and fortunately at the same subject distance as the acquired AF.
Above, 4 shots later, AF holds nicely as would be expected.
Above, a further 3 shots later and still AF holding nicely.
Above, the very next shot and AF starts to shift to the background players!! - Don't ask me why!
Could it have been the vertical stripes on the player's jumper attracted more attention to the AF mechanism?
Above, next shot and strangely AF is lost and has been deferred to the background players - this process actually started in the preceding shot.
Well, you can't always get what you want.
AF with this camera requires a bit of user knowledge and experience in adjusting the settings to suit the style of action.
In this case it was important to ensure foreground obstacles such as players & umpires running past or a head from the crowd in front did not take importance in shifting AF.
In addition, the relatively short focal length meant that the individual player were generally a smaller than normal component of the total image and thus it seems using AF expansion was useful.
Furthermore, setting C. Fn III-2: AI Servo tracking speed = SLOW seemed to make a big difference in maintaining AF on the player instead of diverting to the grass or background crowd/signs.
Given the number of players in a pack and the technical difficulties in working out which player is likely to be of most importance at the time of acquiring AF, I felt that working at f/4 gave better results due to a greater depth of field than at wide open (f/2.8 with tele-extender). The downside is that the background crowd is not as blurred.
The downside with these settings is that if you accidentally acquire initial AF on the background sign or crowd as is easy to do in this situation using a relatively short focal length, the AF tends to stick with the crowd and ignores players running in the foreground! You then have to take your finger off the shutter and re-acquire AF on a player and by then you may have lost the action - so get it right the first time!