I have a doubt about a procedure that I see many photographers continuously apply and they taught me. I see that, when they want to take a picture with their reflex, they do this sequence of steps:
1) point the subject so that it is in the center of the frame (since they keep the center of focus at the center of the view).
2) move the reflex in order to put the subject on one side (in order to apply the rule of thirds). They often call this step "composition".
3) take the picture.
Steps 1) and 2) allow them to focus the subject and to put it in the "correct" third without moving the focus center from the center of the view (which would be a slow operation).
My question is about the focus: how can the reflex keep the focus on the subject if they move it? I do not understand how this is physically possible.