Focus pulls are frequently used to shift the focal point from one part of a scene to another. For example, an actor may be in focus at a distance of 20ft from the camera at the far side of the room and the shot includes a telephone in the foreground which is out of focus. As the phone rings, the actor turns and the focus puller pulls focus so that the phone becomes sharp and the actor goes out of focus. This action is a focus pull.
A properly executed focus pull is a precise and accurate action and is acheived by the focus puller measuring the distance from the camera's focal plane to the objects on the set which are to be in focus. In the above example, the focus puller will know that the actor is 20ft away and that the phone is say 6ft away and have marked corresponding points on the follow focus unit. At the correct point in the action, the focus puller shifts focus precisely from 20ft to 6ft and the phone will be perfectly in focus. The skill of the focus puller is to judge when the actor or the camera, after a dolly move, has not exactly hit their mark and make slight adjustments as necessary.