Rack focus is a cinematography technique that uses shallow focus (shallow depth of field) to direct the attention of the viewer forcibly from one subject to another. Focus is "pulled", or changed, to shift the focus plane, often rapidly, sometimes several times within the shot. On a professional film set, the focus puller, a camera assistant, executes rack focus shots.
The rack focus technique shifts the focus of the scene to provide clarification, new information, or an inner experience of a character.
To change the emphasis of a scene. At the beginning of a shot featuring two characters, the audience might believe the scene is going to be about these characters speaking. By shifting the focus to highlight that another character, who was previously out of focus, is listening in, the emphasis changes.
To provide new story information to the audience. With a focus shift, the audience can see what wasn’t previously apparent: a better view of the setting, or an object that was indistinct before but is now sharply in focus. In this way, filmmakers can guide the attention of the viewers.
To convey the inner experience of a character. During a rack focus, often a character’s facial expressions come into focus, providing critical emotional information.
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