When you look at this photograph, you get a compelling sense of depth—a sense that the various objects in the picture are at different distances from you—despite the fact that you’re really just looking at an array of light specks on your computer monitor, all of which are exactly the same distance from your eyes.
We infer object distances in photos using “monocular” depth cues. No single depth cue is always available or always reliable, but by combining multiple cues, we are usually able to appreciate depth relations pretty well.
Monocular depth cues allow appreciation of depth that require the use of only one eye. The other major depth cue, stereopsis.
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.
Anyone who has ever used the Viewfinder toy, should appreciate how stereopsis works. The Viewfinder presents images taken from two slightly different positions. The small disparity in the two images is used by the brain to create a sense of depth.
Importantly, stereopsis is not present when viewing a scene with one eye. For those patients who lack good binocular vision, stereopsis is not possible because the brain doesn’t get appropriate information to create a sense of depth. In addition, patients with good vision in only one eye, perhaps due to disease, injury, or amblyopia, stereopsis is not possible either.