The focus lock feature of cameras lets you recompose your shots so that you can follow photography
rules, such as the Rule of Thirds.
You can recompose a shot if you lock your focus by pressing the shutter release button halfway down. Move your camera
from side to side to find the framing you like and take your picture with the subject/object still in focus.
Note that you can only move your camera in a plane even with your subject or object. If you
move forward or backward, or if your subject does, you’ll lose focus.
In this image of an abandoned house, I set my focus point on the ripped-up cushion of the second chair, and at that point it was in the middle of the viewfinder. Next, I pressed my shutter release button halfway down, moved my camera to the framing you see in the figure (moved the autofocus point to just below the window) so that the chair’s placement followed the Rule of Thirds, and took the picture.