Its D300. I dont think you can assign anything to it. So my focus is locked on half-press.
why is it called AF-ON then? You'd think theyd name it something simpler. On my camera its just a button labled "C" sometimes i just dont understand those nikons .... ;-)
The AF-ON button on yhe Nikons is the majic button. Alot of people don't even realize every thing it can do. You can assign alot things to it. Check the thread over at the nikoncafe for all the info you'll need.
It's long but a good read