There are two types of depth of field – Deep focus and shallow focus

shallow focus focuses on just the foreground and usually blurs out the background.

deep focus focuses on both the foreground and the background therefore giving a very deep filed of view – showing many layers.

Here is an example of shallow focus from The Social Network (David Fincher, 2010)

Fincher has chosen to focus on the character in the foreground and blur out the characters in the background, this focuses the audiences attention on the character in focus.

This is an example of Deep focus from 12 Angry Men (Sidney Lumet, 1957) and it helps the audience to focus on all the characters in the scene, this is because there isn’t a increase in power or status from any of them – they are equal and therefore are viewed from the same angle.