Looking for my first camera?

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 being focused before the take. Specifically the measurement is from subject to film plane or image sensor, most movie cameras have a small stud even with the film plane where the focus puller will attach his tape measure. Professional 1st ACs have many tricks for pulling focus in difficult situations, such as when accurate measurement is impossible.