In one motion, you press the shutter button halfway down and hold, which causes the camera to engage the autofocus. Once the camera has autofocused on the subject, usually indicated by a beep, you continue to press the button all the way down to take the picture.
While that seems simple and, to people who actually know how to use their digital cameras, foolproof, it’s amazing how many people screw this up. It seems that the majority of individuals that you hand your camera to and ask to take a picture of you with your friends/parents/girl or boyfriend/etc just point the camera and mash the button down. This cause the shutter to fire before the camera focuses, resulting in a blurry picture.
Interestingly enough, these are also the people who give you a look when you review the picture they just took in an attempt to check their work. Then you have to make the decision to ask them to take another picture or just say “thanks” and move on.
So please, for the sake of everyone on a vacation with their loved ones who would like to have everyone in the picture, halfway down to autofocus then all the way down to fire. One motion, no double tapping!