Depth of Field definition

Depth of field refers to the area in focus in front of and behind a plane that is increased as an iris becomes smaller.  The longer a lens is, the less the depth of field is and vice versa.  It does not spread out in a balanced manner; rather, an area is around 2/3 behind the plane of focus and 1/3 in front of it.  It is best to use a depth of field table in order to help take into consideration all the variables that determines the depth of field.  Tables can be found in the American Cinematographer’s Manual.