I use the AF-ON button to focus with my D300, with the AF removed from the shutter button.
I am considering buying a D5100, and I know you can set the back button to AF-ON, but I cannot see a setting to remove AF from the shutter. Is this done automatically when setting the back button to AF-ON? I don't want the shutter to focus at all.
I have searched the internet, only to get conflicting information, and the downloaded D5100 manual is pretty vague.
#1. "RE: Back button focus (AF-On) with the D5100" In response to Reply # 0
Can I ask a fellow Nikonian with a D5100 to just test the setting on their D5100 and let me know the answer. Make sure your camera is set to an AF-C mode, and focuses with the shutter button, then go to Custom setting f2, Assign AE-L/AF-L button. Make a note of the setting you are using, and then set this to the last option AF-ON. This should now allow you to auto-focus with the AE-L/AF-L button, and hopefully remove AF from the shutter button.
There are many advantages to using this setting: Set the camera to AF-C focus mode. Now if you hold down the AE-L/AF-L button you get AF-C. If you press and release you get AF-S, and if you don't press it you have manual. (You may also have to change the shutter priority (option a1) to release for this to work. In addition set option c1 to ON, allowing you to half press the shutter to lock exposure.)
Hopefully I've not missed anything. Try it, especially if you shoot sports or wildlife
#2. "RE: Back button focus (AF-On) with the D5100" In response to Reply # 1
I went out and got my hands on a D5100, and tried out the settings for myself.
Assigning the AE-L/AF-L Button to AF-ON does automatically remove the AF function from the shutter button, as I was expecting. (On the D300 there is a separate setting for removing AF from the shutter.)