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The Function of Rack Focus

by Jeremy Butler

The selective use of focus is facilitated by the photographic phenomenon of depth of field. (Care should be taken not to confuse depth of field with the illusion of depth.)  Depth of field is the distance in front and behind the focus distance that is also in focus (the focus distance being the distance from the camera to the object being focused on). If a lens is focused at 10 feet, some objects nearer to and farther from the camera will also be in focus. This range (say, 8-14 feet in this instance) is the depth of field. Typically, the range is approximately one third in front of the focus distance and two thirds behind it. The camera operator can manipulate depth of field to influence our perception of an image—decreasing the visual impact of parts of the frame by rendering them out of focus and indistinct. A small depth of field—so that only the foreground, middle ground, or background is sharply focused—is termed shallow focus. 

In CSI: Crime Scene Investigation (2000-10), director Nathan Hope has placed a reconstructed vase in the foreground of the image. A book and lab technician Wendy Glenn (Darcy Farrell) behind it are illegibly out-of-focus. The shallow focus of this shot is then manipulated by shifting the focus from foreground to background, from the vase to the book to Wendy. This change of focus is known as racking or pulling focus and in this case it is cued by Wendy’s comments about a missing vase piece. As the shot develops, Hope uses even more rack focusing and some subtle camera movement to bring another character into the scene. As in this instance, rack focus is frequently used to conceal and reveal diegetic information and to add some visual interest to a shot without changing to a new camera position and revising the lighting setup (and costing the producers money).

Rack Focus and Camera Movement in "CSI"

Rack focus and camera movement in "CSI: Crime Scene Investigation."

from CSI: Crime Scene Investigation (2009)
Creator: Jerry Bruckheimer Television
Distributor: CBS
Posted by Jeremy Butler