In optics, particularly photograph and film, the depth of field is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions. Depth of field can be anywhere from a fraction of a millimeter to near infinity. In some cases, such as landscapes, it may be desirable to have the entire image sharp while in other cases, for artistic considerations, may dictate that only a part of the image be in focus, emphasizing the subject while de-emphasizing the background. In cinematography, a large DOF is often called deep focus and a small DOF is often called shallow focus. The intentional use of a shallow focus is also referred to as selective focus or differential focus.