Canon 1D Mk3 front focusing in AI servo mode

[Steve6696]

Hi
I bought a second hand 1d Mk3 for athletics photography that has had the sub mirror fix by Canon. With static objects the focusing is spot on, but with runners coming towards me the focusing is invariably off resulting at f2.8 in the athletes being very slightly out of focus. I estimate the in focus area is an area about 6 inches in front of the athlete (judging by the sharp strip of track just in front of the feet of the athlete).
I use the centre focusing point, evaluative metering, AI servo mode, and the back button to focus. Te lens is a Canon 70-200 f2.8L is ii.
I have used the custom settings at default and also changed a few of them but nothing has yet made a difference.
I know some of these have a bad rep fro focusing but am hoping its something I can fix. Any advice gratefully received.
Thanks
Steve

[dasmith232]

I don't have that camera, but I have the 5D3 which has the same AF subsystem as some of the 1D series. I noticed that sometimes focus was on and other times not with moving subjects. I didn't get as far as you did to correlate the symptoms with certain types of movement.

But I went through the steps to do the micro-adjust on auto-focus and the problems went away. Have you gone through that process before? There are devices you can buy (not cheap) to assist with it, or you can do it for free with a yardstick, a focus target and enough boxes and tape to get everything into proper position.

[Steve6696]

Sorry for not getting back to you. I tried microadjustment without success. Am currently in dialogue with Canon.
Thanks