Photographers sometimes require an extra level of ultra-fine auto focus tuning. Micro adjustment is a feature on newer digital single lens reflex cameras and allows you to fine-tune and register the autofocus position for multiple lens models, then apply that compensation whenever the lens is attached to your DSLR.
It is important to remember that the micro adjustment is an adjustment specific to the camera and lens combination. The adjustment is unique to each lens, and the adjustment information is stored in the camera body.
NOTE: Lenses that do not have an internal focus motor have a larger focus tolerance than lenses with internal focus motors. Because of this, the focus is not always consistent.