Vision

Because our eyes are separated by about 2.6 in (6.5 cm), each eye has a slightly different horizontal view. This phenomenon is called "binocular displacement." The visual images reaching the retina of each eye is a two-dimensional flat image. In normal binocular vision, the blending of these two images into one single image is called stereopsis, which produces a three-dimensional view (one with a sense of depth), and allows the brain to accurately judge an object's depth and distance in space in relation to ourselves and other objects.

Depth and distance perception is also available without binocular displacement and is called monocular stereopsis. Even with one eye closed, a car close to us will appear much larger than the same sized car a mile down the road, or two rails of a railway line appear to draw closer together the further they run off into the distance. The ability to unconsciously and instantaneously assess depth and distance enables us to move about in space without continually bumping into objects or stumbling over steps.