Well, you can approximate back button focus by putting the camera in manual focus mode, setting the AFL/AEL button to only lock focus, and using that. For whatever reason, the focus area rectangle is much bigger when you're in manual focus mode, but you do have the option of fine tuning your focus manually. At least that's the case with the X100S. I've only had mine a few weeks and haven't decided whether it's worth it to use this method regularly or not. I'm thinking it's probably more confusing if I'm switching back and forth a lot, and that the larger focus area would lead to me switching to normal autofocus mode a lot, so I don't think it's what I'll do most often.