Stereopsis (from stereo- meaning "solid" or "three-dimensional", and opsis meaning appearance or sight) is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due to the eyes' different positions on the head. These differences, referred to as binocular disparity, provide information that the brain can use to calculate depth in the visual scene, providing a major means of depth perception. The term stereopsis is often used as shorthand for 'binocular vision', 'binocular depth perception' or 'stereoscopic depth perception', though strictly speaking, the impression of depth associated with stereopsis can also be obtained under other conditions, such as when an observer views a scene with only one eye while moving. Observer motion creates differences in the single retinal image over time similar to binocular disparity; this is referred to as motion parallax. Importantly, stereopsis is not usually present when viewing a scene with one eye, when viewing a picture of a scene with both eyes, or when someone with abnormal binocular vision (strabismus) views a scene with both eyes. This is despite the fact that in all these three cases humans can still perceive depth relations.
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