a3 Focus tracking with lock-on – (1, 2, 3, 4 or 5) This controls how quickly the DSLR refocuses on a new or closer subject after losing focus. It operates in AF-C and AF-A modes. For example you focus on a giraffe that suddenly disappears behind some bushes for a few seconds. Or at a wedding, where a guest walks between you and your subject. If you want to keep focus on the giraffe or guest, set a3 to a high number – i.e. a long time before re-focusing. Alternatively, if you want the AF to quickly refocus, set to a low number (short time).