Lens Focal Length

The zoom lens can be used to quickly choose a wide or narrow field of view for a static shot. It can occasionally also be used during a shot, to follow movement or make a dramatic visual point.

The greater perspective created by using a short focal length creates a stronger sense of depth and a dramatic visual relationship between characters.

The flatter perspective of the long focal length brings distant elements closer, so we see less background.

The resulting images show the effect of both perspective and DOF:

A "focus pull" or "rack focus" changes the focus from one distance to another, shifting the audience's attention.

A smaller aperture (iris) increases DOF, so you normally have more when shooting exterior scenes, and less for interiors.

This shot from The Wire shows the flat perspective and shallow DOF of a long focal length. The characters are emphasized.

This shot shows the strong perspective and deep focus of a short focal length. We see the characters in their environment.

Notice the same red and white sign in the background of both shots.