Depth and Distance

Depth and distance perception relies on a variety of cues our brain receives through the use of both of our eyes. This can cause problems for those of us who only have the use of one eye. While we are able to perceive certain depths and/or distances using only one eye there are other depths/distances that we will not be able to interpret. If you are someone with the use of only one eye you might experience problems judging the distance of a painting hanging on a nearby wall but have no trouble judging the distance of a building or object several blocks away.

When using our vision to judge depth and distances it is up to us to come up with an acceptable estimate. To establish the distance and/or depth we rely on a variety of cues, some of which require one eye and others require both eyes. These cues make it possible for us to estimate distances using only our vision as a guide.

If we were trying to estimate a closer distance (a painting hanging on a wall) we would use Binocular cues. One cue, convergence, is the act of turning the eye inward and occurs when the eyes are focused on a nearby object. The closer the object the greater the convergence.

Another cue is retinal disparity, which is the slight difference between two objects as seen by the left eye and the right eye. Some, including myself, who wear glasses, have slightly different prescriptions for each eye because of the slight difference in the eyes. Because the retinal disparity increases the distance between two objects the brain can use the information to estimate the depth. Since these cues require the use of both eyes to send information to the brain to produce an estimate it is difficult for someone with the use of only one eye to judge distances that are close.

Farther distances, more than 50 feet, rely on different cues. The cues used for greater distances are Monocular cues and they only require the use of one eyes. One cue is interposition, when an object is in between the viewer and a second object and is slightly blocking the view of that second object. Another cue is linear perspective when two parallel lines apear to be coming together and imply the existence of depth. These cues make it possible for someone with only one eye to judge the distance or depth of a building or object that is blocks away.