Also called retinal or visual disparity, it is the small disparity in inputs to the two laterally separated eyes that can contribute to perception of relative depth of nearby objects due to binocular parallax, and thus provides the basis for stereopsis (see figure below). Binocular disparity is usually described in terms of the two differences between the visual angles subtended by points on an object or surface being viewed, with the visual system being capable of responding three-dimensionally to retinal disparities as small as 2 seconds of arc corresponding to 1/1800 of a degree. The idea that binocular disparity contributes to depth perception originated with the invention of the stereoscope by Charles Wheatstone (1802-1875) in the 19th century, which he used to show that the brain uses horizontal disparity to estimate the relative depths of objects with respect to a fixed point, a process known as stereopsis.
