I have started to use my D3100 with my AE/AF-L button set to AF. So in other words I use this button instead of the half press shutter button to AF. I'm new to this technique and came across it here at the Canon website
A problem with the D3100 is that the shutter will not activate if focus is not locked on. If I'm in AF-C using a single focus point and I want to recompose (as in point 1 of the article above) I should be able to just lift my thumb off the AE/AF button after focusing on my subject. Moving the camera slightly to the side I should be able to take the shot but I can't. It wont allow me to release the shutter as the new centre point is out of focus. To get this working I need to be in AF-S (which is fine) and then HOLD the AE/AF button while recomposing and taking shots. Does this not miss the point of the back button AF? In that article the author talks about taking his finger off the back button once focused and then shoots freely. Thats an advantage to me. Keeping my finger down is not. I only want to focus when that back button is pressed and to be allowed to shoot even if the new center point is out of focus.