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A focus puller, or 1st assistant camera operator, is a member of a film crew’s camera department whose primary responsibility is to maintain image sharpness on whatever subject or action is being filmed.
“Pulling focus” refers to the act of changing the lens’ focus distance setting in correspondence to a moving subject’s physical distance from the focal plane. For example, if an actor moves from 8 m away from the focal plane to 3 m away from the focal plane within a shot, the focus puller will change the distance setting on the lens during the take in precise correspondence to the changing position of the actor. Additionally, the focus puller may shift focus from one subject to another within the frame, as dictated by the specific requirements of the shot (cinematic techniques).
A good focus puller will have an intimate knowledge of cinematographic and optical theory. Depending on the parameters of a given shot, there is often very little room for error. As such, the role of a focus puller is extremely important within the realm of a film production; a “soft” image will, in most circumstances, be considered unusable, since there is no way to fix such an error in post-production. One must also consider that an actor may not be able to duplicate his or her best performance in a subsequent take, so the focus puller is expected to perform flawlessly on every take. Because of these factors, most production personnel consider the focus puller to have the most difficult job on set.Retrieved from "http://en.wikipedia.org/wiki/Focus_puller"
Today I learned that there are many, many people, measurements, time and work involved in making sure that films stay in focus. It's a lot more complicated than I thought and explains long filming schedules and budgets. Amazing!