While a lens focuses on a single plane of depth, there is usually an additional area in focus behind and in front of that plane. This is depth of field. Depth of field increases as the iris is closed. There is more depth of field the wider the lens and less the longer the lens. There is a deeper area in focus the further away a lens is focused than there is when a lens is focused close. Depth of field does not spread out evenly; the entire area is about 1/3rd in front and 2/3rds behind the plane of focus. To factor together all these variables it is best to consult a depth of field table, such as the ones found in the American Cinematographer’s Manual.