Trap focus on half shutter only

[pholler]

Hi guys!

I recently upgraded from the 5D2 to the 5D3 and found the trap focus feature extremly powerful on the Mark III. It was unaccurate with the Mark II AF-system but works unbelievably wonderful with my MF-lenses on the Mark III.

Most of the time i photograph kids and for that i want to prefocus with AF-assist and use the trap focus feature only when things go really fast. For that it would be wonderful to switch fast between trap focus and the normal operation.

Is it possible to use the AF-on-button for AF-assist and the half-shutter for trap-focus? I know it's possible to recognize the half-shutter as you already use double-half-shutter for AETTR.

Thanx!

Peter

[pholler]

No ideas?