| | The center-to-center distance between an adult person's eyes averages between 58 to 72mm, and is described as the Interpupillary Distance (PD). The binocular should be capable of any setting between those two measurements. When the user makes this adjustment correctly, the sight picture forms a single round circle. When it's slightly out of adjustment you can see two distinct circles, as it is often depicted in the movies. This is corrected by folding in or separating the barrels of the binocular. All binoculars are hinged at the center for this purpose.
Generally speaking, roof prism binoculars have a smaller range of hinge adjustment than do porro prism binoculars. |  | |