English: Hooded post viewed through an aperture of the rotating drum rear sight on a Heckler & Koch MP5.
The rotating drum sight apertures on Heckler & Koch sub machine guns have various diameter apertures to optimize the sight line for various ambient light conditions. The theory of operation behind the aperture sight is often stated that the human eye will automatically center the front sight when looking through the rear aperture, thus ensuring accuracy. An additional benefit to aperture sights is that smaller apertures provide greater depth of field, making the target less blurry when focusing on the front sight.