I purchased my 20D in September and all went OK until I began using the Canon 600mm F4 IS lens. The camera constantly front focused using both the AF or MF mode. My 100-400 IS appears to be fine.
By accident last week I was talking to a press photographer friend who, having purchased a 1d Mk11 had the same problem with all his long lens when using his camera. After a lot of discussion with Canon UK, all the equipment was returned and after several false starts, the gear was "re-calibrated" and now works fine.
Intersestingly, when a friend tried his 600mm IS on my 20D camera the focus problems did not occur. However, when I then used my lens on his 20D camera the same problem occured. This appeared to indicate that the lens, not the camera was at fault.
I was sceptical, but contacted Canon UK and arranged to drop of my camera and lens and wait in the car park after a 500 mile round trip to get the gear sorted out prior to leaving for Poland on a wildlife photo shoot this coming Saturday. The re-calibration worked out perfect and the two items now function together as they should. I was informed whilst at Canon, the company may have to consider re-designing the lens mount on a number of their digital cameras to resolve this unusual problem. I have yet to try my 600mm lens on a 35mm film camera body to establish if the lens still functions correctly with a SLR??
To check the problem with the 20D, or any other Canon DSLR is easy, just photograph a calibration charts place at 45 degrees to the horizonatal camera plane, whilst setting the camera apperature wide open, in my case f4, from the lens minimum focal distance, focus on the centre point of the chart. If the camera is "front fucusing" as mine was, the lines marked +1,2,3, (closest to the lens) at the near side of the charts centre spot will be sharp.
If the image shows that between +1 / -1,-2,-3 are sharp, your equipment is OK and does not require calibration. If you try this out use a high shutter speed,