Editor: The term “diopter” and “diopter sight” is used to describe an optical rear sight with a circular aperture. By common usage it does not have to be a glass lens element (although the term “diopter” lens is used often to describe adjustable ocular lenses in cameras and other optics). Adjustable aperture rear sights, aka “Diopter Rear Sights” are commonly used on competition rifles. See: http://en.wikipedia.org/wiki/Diopter_sight

“Old eyes” that can no longer focus on the front sight need a lens, (a “diopter”), to recover that focus. Both young and old eyes can benefit from an aperture near the eye to improve the rear sight image.