James Brandon:

If you stand 4 feet from your subject and point the camera up at the subjects face, then you are no longer 4 feet away from what you’re focusing on. If the length from your camera to your subjects chest is 4 feet and the length from your subjects chest to their eye is 2 feet, then the length from your subjects eye to your camera is 4.5 feet. Are you getting this!? That means that if you focus on your subjects eye, move the camera down to their chest to recompose, then your focal plane is now half a foot behind your subject!

Makes perfect sense, so get used to moving your focus points on your camera.

Posted by Ben Brooks