Still learning about my new D7000. I've come across assigning buttons to frequently used functions. It appears that there are three different buttons that can used. 1) FN function button, 2) Preview button, and 3) the AE-L/AF-L button. Could I solicit the help of any of you who have in fact assigned these buttons. And ask, what have you assigned the above #1 #2 and #3 buttons too, and possibly why.