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Can someone, without emotion or smart remarks, please explain what using the AF On button does that using the shutter release does not do?
Exactly what I stated earlier: it decouples the mechanism of focusing, from the mechanism of releasing the shutter.
That means you can release the shutter without forcing the camera to refocus every time. You CAN have the camera refocus constantly, by holding down AF-ON, but you can also choose to release the shutter without triggering autofocus.
Additionally, you can leave AF-C enabled all the time, because you can half press the shutter to engage VR and metering, without having the camera constantly adjusting its focus. You press AF-ON, and the camera focuses. If you release AF-ON then, it is behaving just like single focus AF. If you want continuous AF, you just hold down the AF-ON button.
If you've tied focus to the shutter button, you can't have the shutter button half-engaged, thus triggering VR and metering, without the camera constantly refocusing.