Canon EOS 5D Mark III Instruction Manual (en)

 Fine Adjustment of AF’s Point of FocusN
When [  Clear all] appears at the bottom of the screen, pressing the 
<L> button will clear all the adjustments made for [All by same 
] and [Adjust by lens].
Notes for AF Microadjustment
Clearing All AF Microadjustments
The AF’s point of focus will vary slightly depending on the subject 
conditions, brightness, zoom position, and other shooting conditions. 
Therefore, even if you perform AF Microadjustment, focus may still not 
be achieved at the suitable position.
The adjustments will be retained even if you clear all the camera settings 
(p.56). However, the setting itself will be [Disable].
It is best to make the adjustment at the actual location where you will 
shoot. This will make the adjustment more precise.
Using a tripod when making the adjustment is recommended.
For checking the adjustment result, shooting at the 73 image-recording 
quality is recommended.
The adjustment amount of one step varies depending on the maximum 
aperture of the lens. Keep adjusting, shooting, and checking the focus 
repeatedly to adjust the AF’s point of focus.
AF adjustment is not possible for the Live mode and u Live mode 
(contrast AF).