This document discusses key concepts related to lenses, focal length, and depth of field in filmmaking. It explains that focal length impacts the field of view and depth of field, with longer lenses having a narrower field of view and shallower depth of field. Focus determines what is sharp in the frame. Focal length, focus, and f-stop or aperture are variables of a lens. Different examples from films are provided to illustrate the effects of focal length on compressing or expanding space as well as manipulating depth of field. The document also discusses achieving a sense of increasing claustrophobia through gradually shifting to longer lenses and lowering the camera angle in the film 12 Angry Men.