RULES OF CINEMATOGRAPHY
Depth of Field
Depth of field refers to the area of a shot that is in focus. A picture with a deep depth of field has almost everything in focus. A shot with a shallow depth of field has just a small portion of the picture in focus.
Depth of field forces the viewer to look where the shot is in focus. So images in the background that are blurred will not be noticed. Only the objects that are in full focus will get the viewer's attention.
Rack focusing is a technique used by cinematographers to change of area of a picture that is in focus in mid shot. The shot may start off with one character in focus and then switch to the other one when they start talking. When you rack focus, you can change where the viewer is looking because you have changed the focus. This works really well in conversations and in situations where you may want to reveal something significant.
A sample of rack focusing can be seen as part of the video on the right.
A rack focus in filmmaking and television production is the practise of changing the focus of the lens during a shot. The term can refer to small or large changes of focus. If the focus is shallow, then the technique becomes more noticeable. In professional films, a camera assistant called a focus puller is responsible for rack focusing
Shallow Depth of Field
Deep Depth of Field