James,
It sounds like you need to adjust the focus calibration for this lens to your camera. Here is a link to Canon's micro focus adjustment guide: https://cdn.static-bl.com/images/manual/Canon-AF-Micro-Adjust-Guide.pdf
It is a simple process but I use a slightly different procedure to speed things up. I follow the Canon process EXCEPT that I take multiple shots at different offsets in a single test run so that when I examine them on the computer I will either have the exact calibration offset in the set or will know exactly the range that I need to zero in on.
Once I have the target set up, I take images with the micro-adjust set at 0, +3, +5, +8, -3, -5, and -8 and then check on the computer to see which of these is exact or closest.
A yardstick or similar set in the grass makes a good target and the grass blades provide a clear example of where the optimal focus range is located. With a zoom lens, there is a calibration at the wide and tele ends and I would do all of the test images at both ends (a total of 42 test shots in your first run since you will take 3 shots each at each offset for both the 70 and 200 ends).
Shoot in manual mode with the lens wide open (f2.8), single shot AF, with enough shutter speed to avoid shake (in daylight 1/500 or greater should be possible) and set the ISO to auto. Take three shots at each setting just to ensure that you don't make a judgement on a one shot aberration in the AF performance. The micro adjustment offset is contained in the EXIF data so you can clearly identify the offset when looking at the image in DPP.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
