http://www.cleaningdigitalcameras.com/pdf/EOS-1DMarkII_IM-EN.pdf![]()
I use * to focus and the assist button next to it to focus the registered AF point so if you register a point you can do that too. This way I can quickly switch between two AF points which I use all the time.
Press AF select and FEL at the same time to register an AF point to the assist button and change CF 18 to 2 to have it momentarily use the registered point while you hold the assist button, it will focus at the same time if you have * focusing enabled with CF 4 to 1 or 3 (I prefer 3).
Can the Mark III do this, switch between both points and do AF when you press/hold them? I'm not sure it can, you switch between regular and registered points with the multicontroller on the III, right?
You could always use CF IV 2 on the III to swap the * and AF-ON buttons
I can do that with my 40D to make my 4 bodies the same (The Rebel T1i is like the 1DII, it doesn't have AF-ON, just * and AF select).












