hi, i understand this has been asked many times. but i still can't find a clear answer to my doubt. when i set my fn to focus, then when i depress the fn button, a green light appear. so should i keep holding(depressing) the button and take my shot or i can now remove my fn button once the green light appear? by holding the fn button down, is it continuously re-focusing like AF-C or does it just show that i have focus(once) only? to add on, should i use a single box or 9 box for focusing? mainly shooting portrait(full body). please advise, thank you.