#1. "RE: Back Button focus on D3100 question?" In response to Reply # 0
Change the fuction of the AE-L/AF-L Button to AF-ON. That will decouple the AF fuction from the shutter button and transfer it to the AE-L/AF-L button. See Page 146 of the D3100 Users Manual. It will work the same way on the D3100 as it does on the D300. Good Luck and Enjoy your Nikons!