1st AC, or focus puller has one main job: they sit next to the camera during filming and operate the lens’s focusing ring. The role of the focus puller is to keep the right subject in focus throughout each scene.
Pulling focus is considered one of the hardest jobs on set—the first AC not only has to get the shot in focus, they have to keep the shot in focus. For example, if an actor walks from the background to the foreground, the first AC has to calculate exactly how much to pull the focus in order to keep the actor sharp in the focal plane.
The first AC is also in charge of maintaining and organizing all camera equipment and accessories on set—cleaning camera lenses and setting up and taking down the camera each day, so that it’s ready for the next day of filming.