If the lens is focusing properly at shorter focal lengths but is placing the point of sharpest focus just in front of or behind your intended target at 70mm, with the 5D Mark III you can adjust the focus calibration on the telephoto end of the lens without moving the calibration at the wide end.
Focusing errors are usually less noticeable at wider focal lengths than at longer ones, though, so be sure to check both ends and an intermediate focal length or two to be sure when you do the AFMA adjustment. Intermediate focal lengths will be interpolated between the two extremes. So if what you do on the long end pulls focus away from where you like in an intermediate focal length, an adjustment of the wide angle setting in the other direction will help with the middle focal lengths.
To do AFMA adjustment for the telephoto end of the lens, go to:
- AF (purple) menu tab 5
- Scroll down to 'AF Microadjustment' and press the 'Set' button
- Scroll to 'Adjust by lens'
- Press the 'Info' button
- Scroll to 'T' (for 'Telephoto')
- Press the 'Set' button
- Use the 'Quick Control Wheel' on the back of the camera to change the value. If the lens is focusing behind your intended target, set a positive number by moving the cursor to the right (or increase the value if a number other than '0' is already selected). If the lens is focusing in front of your intended target, move in a negative direction to the left.
- When the value you desire is displayed, press the 'Set' button
- Test and repeat until you have it dialed in to your satisfaction. Use the lens' maximum aperture when testing to get shallowest DoF that will help you see differences more clearly.
The wide angle adjustment is similar, except you scroll to the 'W' line.
For how to set up your camera to do accurate AFMA measurements and adjustments, please see:
Which offers better results: FoCal or LensAlign Pro?
What is the best way to micro-adjust a camera body to a particular lens?
For related questions that deals with diagnosing AF problems when shooting portraits with wide apertures, please see:
Focus point causing unsharp images?
How do I diagnose the source of focus problem in a camera?
For some of the things you might need to consider when moving from a narrower aperture, IS lens to a wider aperture, non-IS lens like the EF 24-70mm f/2.8 L, please see:
Canon 7d & 24-70 ii - can't get a crisp or well exposed shot
For other considerations concerning the EF 24-70mm f/2.8 L , please see:
Canon 24-70mm 2.8f - Optimal aperture for sharper pictures
For help understanding how AF systems with large number of AF points work, please see:
How can I more consistently focus on the point I want?
Pictures of surfers start in focus then go out of focus?