Hey John, you can assign it to fn2, I just did. First you have to set a custom button setting. Go to the IQ menu, scroll to Edit/Save Custom Setting, select the number you want, C1, C2, whatever, then pick MF, focus assist Off. Then go back to the Custom Button menu, hold down Disp/Back until it comes up. You can now assign your custom setting to button Fn2. It works perfectly as a MF Focus Peak on/off button and reacts immediately. Thanks for bringing this up, it is nice to have and I didn't think about it before.