Focus puller
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In cinematography, a focus puller or first assistant camera (1st AC) is the member of a film crew responsible for keeping the camera's focus at the proper position during a shoot. This typically requires pulling the focus with a follow focus device during the take, to compensate for camera and/or subject movement. This requires intimate knowledge and experience with the optical principle of depth of field, which the 1st AC must learn to exploit to his or her advantage. In extreme circumstances the depth of field may sometimes be very small, as little as 1/4 or 1/8th of an inch (3–6 mm). Since 1st ACs do not look through the camera (that is the job of the camera operator) and thus cannot see the results of their focusing in real-time, this job is considered to be extremely technically difficult.
To accurately focus on the subject, it is common to measure the distance (usually with a tape measure or, more recently, with electronic tape measures using lasers) between the camera and the subject before the take, done usually as the rest of the film crew prepares for the shot. Specifically the measurement is taken from the subject to the film plane or image sensor. Most movie cameras have a small stud level with the film plane where focus pullers attach their tape measure. It is important for the 1st AC to interfere with the workings of the set as little as possible; while it is often common to wait for lighting setups to be completed or for actors and directors to hone the scene, the camera crew should pride itself on never being the cause of a slowdown in shooting.
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 when it is impossible to accurately measure the distance.
It is also the focus puller's responsibility to maintain the camera through the duration of the filming period, to apply or remove any accessories (such as lenses, filters, matte boxes and external viewing monitors), to reload the camera, to keep the camera, lenses and film gate clean, and to be aware of lens flares. Focus pullers also oversee the 2nd assistant camera (also known as the "clapper loader") and any other members of the camera assist team.
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