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A focus puller or first assistant camera (1st AC) 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" or "rack focusing" refers to the act of changing the lens's focus distance setting in correspondence to a moving subject's physical distance from the focal plane. For example, if an actor moves from 8m to 3m away from the focal plane within a shot, the focus puller will change the distance setting on the lens during the take in precise relation 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.
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, some production personnel consider the focus puller to have the most difficult job on set.
The 1st AC (or Focus Puller) has one of the hardest jobs on the set. And it's one of those jobs that are never noticed until it is wrong. Then you get an almighty bollocking, or you get fired ... Focus Pulling not only involves what it sounds like, but also the Focus Puller "runs" the department, in the sense of taking care of all the camera gear, and making sure that everything is tickety-boo. I have my own camera, so it's treated very well! A focus puller relies heavily on the Operator to tell him if the shot is out of focus — after all only the operator is actually looking through the lens.
During production, 1st ACs are also responsible for the maintenance and upkeep of all camera equipment and accessories, including; lenses, filters, batteries, film magazines/recording media, matte boxes, etc., as well as configuring the camera and its accessories in multiple ways for different setups. 1st ACs arrive on set before the director, director of photography (DoP) and camera operator, and ensures the camera and all required lenses are prepared for the day's shoot. During filming, the 1st AC is responsible for moving the camera to each setup as directed by the Director of Photography. On larger budget productions, the camera is usually mounted on a dolly for most of the day, which is operated by the Dolly Grip. At the end of each shooting day, 1st ACs clean the equipment and pack it up in preparation for the next day. If there is a problem with the rushes (such as a scratch on the film), the focus puller communicates with the film lab to fix any faults with the camera or film stock.
Sharp focus is fundamental to reproducing a realistic, appealing image, and a viewer's attention is automatically drawn to sharper areas. When done correctly, good pulling goes mostly unnoticed by the audience.
The shooting of a scene typically begins with a blocking rehearsal, in which the various actors' positions are established. During the rehearsal, the 2nd AC lays tape marks on the floor at all points an actor pauses movement. The actors then leave set to go through hair and makeup, and stand-ins come in to take their places at these various positions for the purposes of lighting, framing, and focus-mark setting. Once a camera position is established by the director of photography and camera operator, the 1st AC begins to measure the various distances between the actors' marks and the focal plane of the camera. These distances are recorded in a series of grease pencil/pen marks on the focus barrel of the lens, and/or the marking disc on the follow focus device. Using the stand-ins or 2nd AC, marks are checked through the viewfinder and/or the onboard monitor for accuracy. Marks may be repositioned in order to provide specific framing desired, and the 1st AC must be aware of this and re-measure/re-set his marks accordingly. Additionally, the 1st AC may have the 2nd AC lay down specific distance marks on the floor which will be referenced during the take as actors move between their marks, in order to assist in accurately adjusting the focus to the correct intermediate distances.
When the actors return to set, there is usually a rehearsal for camera in which the focus puller and operator will practice the shot and make sure everything has been set up properly. During a take, the focus puller modifies the focus based on the dialog, movement of the actors or subject, movement of the camera, the DoP's directions, and compensates on the fly for actors missing their marks or any unforeseen movement. In cases where an obstruction prevents the focus puller from seeing all his marks, he may request the 2nd AC call the marks for him over a 2-way radio during the shot. In some situations, such as on long lenses, wide apertures, very close distances, or any combination of the three, a subject moving even a few millimeters may require immediate and very precise focus correction.
After a take, if the focus puller feels he's made a mistake—be it a timing error, a missed mark, or any other issue which may have rendered some part of the take "soft," he or she will typically report this to the operator (who most likely noticed the error in the viewfinder) or director of photography, and may ask for another take if another wasn't already planned.
Traditionally, the focus puller used only his marks and his own well-developed sense of distance estimation to achieve good results. Over the last decade, the increased use of digital cameras, higher-resolution video taps and/or on-camera monitors have provided focus pullers with additional tools to help maintain proper focus. A high-definition monitor can be particularly useful when a fast-paced production simply does not allow time for the focus puller to set and check all marks that may be needed, or if no rehearsal will be provided.
Professional 1st ACs have many tricks for pulling focus in difficult situations or when accurate measurement is impossible. Often, before a scene is even rehearsed or established, the 1st AC will take surveying measurements of the general environment in order to have a good idea of the distance between reference points, such as patterns on the floor or walls, furniture, and whatever else might be around. These reference measurements can be used to quickly establish rough distances between the camera and the subject in chaotic shooting circumstances.
In addition to keen eyesight, reflexes, and intuition, the focus puller's primary tools are a cloth or fiberglass tape measure, steel tape measure, laser rangefinder, and in some cases an on-camera ultrasonic rangefinder which provides a real-time distance readout mounted on the side of the mattebox or camera body. In setups where the focus puller cannot touch the camera, such as on steadicam or crane shots, he or she will use a remote follow focus system, though some focus pullers prefer using a remote system at all times.
At the end of a successfully shot scene, the focus puller will be called upon to "Check the gate". This involves removing the lens and inspecting, with the aid of a flashlight and magnifying glass, the aperture to which the film is held during exposure. The focus puller is looking for any foreign bodies, e.g. hair, bits of broken film, fluff etc., that would show on the film and therefore ruin the scene that has just been filmed. This obviously is only relevant to shooting on film: however, a similar task is required for digital movie cameras (although not nearly as often) which involves checking the sensor.
The 1st AC reports to the director of photography, works alongside the camera operator, and oversees the 2nd assistant camera (also known as the "clapper loader") and any other members of the camera department.