Let's use an example... Your camera/lens combination is back-focusing by about half an inch. Since the "0" line falls within the depth of field, the manufacturer will call this "in spec". And, in the real world when shooting at further distances, the DOF will be larger and pick up the slack. The problem arises when the AF mis-triggers on some percentage of your shots. If the AF glitch forces a greater back focus, your desired point of focus will be even further out. However, if the glitch forces more of a front focus, the "mis-calibration" will work in your favor. When you adjust the AF to be centered on the "0" line, you have a 50/50 chance of any AF glitch working for you or against you.
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