Depth of field is defined as the area in a projected image, forward and aft of the focal plane, which also appears to be in focus in the image. When you pass light through a lens and focus that light to form an image on a piece of film, digital sensor, projection screen, etc., the area of the image that is in true focus is thin—the focal plane. Everything else is out of focus, to some degree. However, because of the delicacy of the out-of focus regions, you cannot notice the softness of the image until, as objects are located farther away from that plane of focus, the blur reaches a certain level. Our eyes, and cameras, see a region of depth in an image where everything appears to be in focus.
