In optics, particularly as it relates to film and photography, the optical phenomenon known as depth of field (DOF), is the distance about the plane of focus (POF) where objects appear acceptably sharp in an image. Although an optical imaging system can precisely focus on only one plane at a time, the decrease in sharpness is gradual on each side of the POF, so that within the DOF the unsharpness is imperceptible under normal viewing conditions.
In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, emphasizing the subject while de-emphasizing the foreground and background. In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus.