Retinal disparity is the slight difference between the images projected on the two retinas. Because the eyes are separated horizontally, each receives a somewhat different view of the world; the visual system uses that difference as a binocular cue for depth.

Nearby objects produce larger disparities than distant ones. Neurons in the visual cortex are tuned to specific disparities and signal the corresponding depths. The cue is especially precise within a few meters and becomes less informative at greater distances, where monocular cues take over. Stereoscopes, 3-D films, and virtual-reality displays exploit retinal disparity by presenting slightly different images to each eye.

People who lack binocular vision from early childhood often develop little stereo acuity and rely more heavily on monocular information. Temporary disruption of disparity, as with certain optical devices, can impair fine depth judgments. Retinal disparity is the geometric foundation of stereoscopic depth perception.

The small mismatch between the two eyes’ views is what allows the brain to compute precise near depth.