Originally Posted by
Manfred M
The fine tuning process is used to tweak your autofocus system so that it correctly focuses your lens(es).
The "best" way of ensuring 100% accurate focus is to set your focus manually on an enlarged view of the subject where you want sharpest focus, either magnifying and using LiveView on the back of your camera or shooting tethered and using a larger monitor screen to do your focusing. The latter is the standard way that professional video and film shooter focus their cameras. If you ever visit a production site, they generally have a large hooded screen where this is all done.