The trick itsto accurately determine the value to enter to make things better not worse. The adjustment works by the photographer entering a value in to change the point that is in focus. To fine tune the focus point both Canon and Nikon have a custom function (AF Fine-Tune Adjustment for Nikon and AF Microadjustment for Canon). Very small variances in either the sensor positions or the autofocus motor can cause radical differences in the focus point. The autofocus system detects the difference in the waveforms of one sensor to the next and is able to control the point of focus based upon this difference by controlling the autofocus motor in the lens. The autofocus mechanism used by DSLR camera's involves sensors in the mirror box.
