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.