Canon 1D Mark III autofocus
First I thought I would try the EF 135mm f/2.0L with 1.4x EX II extender in AI Servo with AF setting to ignore foreground obstacles and used ISO 3200 to give 1/1600th sec to minimise camera shake or subject movement causing unsharpness.
I used the AF-ON button to set focus whilst attempting to ensure the subject was on the center AF point to acquire focus
This whole sequence ended up the same - nicely in-focus advertisement banners in the background but out of focus players.
I am guessing the main problem here is that unless the player fully covers a AF point then the AF point selects the background.
This may be remedied with the new firmware upgrade.
Worked out how to get 100% of the sequence in focus - use manual focus with a $50 lens from Ebay, the Sigma 70-300mm f/4.5-5.6 for Olympus OM mount - here I have used ISO 6400 to cope with f/5.6 at 300mm:
the whole of this sequence was in focus BUT it required pre-focusing on a predicted zone of action.
Unlike the AF, there is no evidence of players going in and out of focus as with the current AF mechanism.
An advantage of the manual focus method is that even if the players fall out of the AF sensor ring, they still remain in focus:
and another example of manual focus:
BUT there is a big problem with manual focus - if the action quickly moves to a different zone, you will miss shots as manually acquiring new focus takes time.
I went back to the AF lens and had mixed results - much more reliable when the action was close and filled much of the central region, very unreliable when players at 60m as with sequence 1 above, presumably as dominant central AF point selects background or foreground in preference to the players.
A solution (assuming no improved algorithm from Canon) in this scenario would be to get closer to the action with a 300mm f/2.8 or even a f/4 lens so the players fill the central AF region more completely.