Focus puller

In cinematography, a focus puller or first assistant camera (1st AC) is a member of a film crew's camera department who is responsible for keeping the camera properly focused during a shoot.
Sharp focus is elemental to reproducing a realistic, appealing image, and a viewer's attention is automatically drawn to sharper areas. When done right, good pulling goes mostly unnoticed by the audience, but soft focus is distracting, nearly impossible to repair after the fact, and can ruin a take. Focus pullers are therefore expected to perform flawlessly every time.
To prepare for a take, the focus puller first measures the distances during rehearsals, sets reference marks with the help of the 2nd AC, compares them with the distance markers on the particular lens being used, and marks them on his/her follow focus ring. During a take, he/she modifies the focus based on the dialog, action, the DP's directions, and compensates on the fly for actors missing their marks or any unforeseen movement. In some situations, an actor's head moving even a few millimeters may require instantaneous focus correction.
Traditionally, the focus puller does not look at the recorded image to do his/her job; using the marks instead of just looking through the viewfinder produces far more reliable and repeatable results. With his/her position beside the camera he/she can see his off-frame marks, and also gain a three-dimensional view of the scene, critical for judging distances. This method evolved with film cameras, which have only one sharp viewing apparatus - taken by the camera operator. With the advent of digital video cameras and increasingly reliable LCD monitors, focus pullers do sometimes check their work on the screen.
Besides his/her eyes, the puller's main tools are a follow focus device and a distance measuring tool - usually with a tape measure or, more recently, with electronic tape measures using lasers (some discourage the use of lasers due to a potential liability resulting from damage that the light might inflict on an actor.
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.