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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 the camera lens's optical focus on whatever subject or action is being filmed.
"Pulling focus" refers to the act of changing the camera lens's focus distance to a moving subject's distance from the focal plane, or the changing distance between a stationary object and a moving camera. For example, if an actor moves from 8 meters to 3 meters away from the focal plane, the focus puller changes the lens's distance setting in precise relation to the actor's changing position. The focus puller may also shift focus from one subject to another as the shot requires, a process called "rack focusing". 
There is often very little room for error, depending on the parameters of a given shot. The focus puller's role is extremely important to a film production. In most circumstances, a "soft" image is considered unusable, as such an error can't be corrected in post-production[ citation needed ]. An actor may not be able to duplicate their best performance in a subsequent take, so the focus puller is expected to perform flawlessly on every take.
British cinematographer Oliver Stapleton has written:
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. 
Once a camera position or overall "shot" 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 or pen marks on the focus barrel of the lens; similar marks may be placed on the marking disc or ring of any follow focus device being used. Traditionally, the focus puller used only their marks and their 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 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.
The 1st AC may take surveying measurements of the general environment in order to have a good idea of the distances 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.
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.
The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera.
A film crew is a group of people, hired by a production company, for the purpose of producing a film or motion picture. The crew is distinguished from the cast, as the cast are understood to be the actors who appear in front of the camera or provide voices for characters in the film. The crew is also separate from the producers, as the producers are the ones who own a portion of either the film studio or the film's intellectual property rights. A film crew is divided into different departments, each of which specializes in a specific aspect of the production. Film crew positions have evolved over the years, spurred by technological change, but many traditional jobs date from the early 20th century and are common across jurisdictions and filmmaking cultures.
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
A camera is an optical instrument that captures images. Most cameras can capture 2D images, while some more advanced models can capture 3D images. At a basic level, most cameras consist of a sealed box, with a small hole that allows light to pass through and capture an image on a light-sensitive surface. Cameras have various mechanisms to control how light falls onto the light-sensitive surface, including lenses that focus the light and a shutter that determines the amount of time the photosensitive surface is exposed to the light.
A view camera is a large-format camera in which the lens forms an inverted image on a ground-glass screen directly at the film plane. The image is viewed and then the glass screen is replaced with the film, and thus the film is exposed to exactly the same image as was seen on the screen.
The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object.
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.
In photography and cinematography, a wide-angle lens refers to a lens whose focal length is substantially smaller than the focal length of a normal lens for a given film plane. This type of lens allows more of the scene to be included in the photograph, which is useful in architectural, interior and landscape photography where the photographer may not be able to move farther from the scene to photograph it.
Cinematography is the art of motion picture photography.
A camera operator, or depending on the context cameraman or camerawoman, is a professional operator of a film camera or video camera as part of a film crew. The term "cameraman" does not imply that a male is performing the task.
In photography and cinematography, perspective distortion is a warping or transformation of an object and its surrounding area that differs significantly from what the object would look like with a normal focal length, due to the relative scale of nearby and distant features. Perspective distortion is determined by the relative distances at which the image is captured and viewed, and is due to the angle of view of the image being either wider or narrower than the angle of view at which the image is viewed, hence the apparent relative distances differing from what is expected. Related to this concept is axial magnification -- the perceived depth of objects at a given magnification.
A follow focus is a focus control mechanism used in filmmaking with film cameras and in television production with professional video cameras. It is ergonomic rather than strictly necessary; in other words it does not contribute to the basic functionality of a camera but instead helps the operator be more efficient and precise. It is usually operated by a focus puller but some camera operators prefer to pull their own focus.
A clapper loader or second assistant camera is part of a film crew whose main functions are that of loading the raw film stock into camera magazines, operating the clapperboard (slate) at the beginning of each take, marking the actors as necessary, and maintaining all records and paperwork for the camera department. The name "clapper loader" tends to be used in the United Kingdom and Commonwealth, while "second assistant camera" tends to be favored in the United States, but the job is essentially the same whichever title is used. The specific responsibilities and division of labor within the department will almost always vary depending on the circumstances of the shoot.
In filmmaking and video production, a shot is a series of frames that runs for an uninterrupted period of time. Film shots are an essential aspect of a movie where angles, transitions and cuts are used to further express emotion, ideas and movement. The term "shot" can refer to two different parts of the filmmaking process:
Macro photography is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is greater than life size . By the original definition, a macro photograph is one in which the size of the subject on the negative or image sensor is life size or greater. In some senses, however, it refers to a finished photograph of a subject that is greater than life size.
An autofocus optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished as active, passive or hybrid types.
A film plane is the surface of an image recording device such as a camera, upon which the lens creates the focused image. In cameras from different manufacturers, the film plane varies in distance from the lens. Thus each lens used has to be chosen carefully to assure that the image is focused on the exact place where the individual frame of film or digital sensor is positioned during exposure. It is sometimes marked on a camera body with the 'Φ' symbol where the vertical bar represents the exact location.
The film gate is the rectangular opening in the front of a motion picture camera where the film is exposed to light. The film gate can be seen by removing the lens and rotating the shutter out of the way. The film is held on a uniform plane at a calibrated distance in the gate by a pressure plate behind the film.
Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the camera and is therefore sometimes referred to as "lens-to-film tolerance".
Tilt–shift photography is the use of camera movements that change the orientation or position of the lens with respect to the film or image sensor on cameras.