Modified: 2020-03-18
One of the major perceptual tasks is judging depth in a visual stimulus, or, being able to tell which objects are closer to you from those that are further away. This task is accomplished many ways. One way is via binocular cues for depth perception, or cues that require the use of both eyes. The binocular cues are more powerful than the monocular cues. You can prove this to yourself by trying to perform a task that requires depth perception, for example, shooting a basketball. You should be more accurate in the long run if you shoot with both eyes open. In the lab, special tests demonstrate the superiority of binocular cues.
Retinal disparity is the major binocular cue. Each eye sees a slightly different image. You can demonstrate that by holding a pencil at arm's length in front of you. Then pick a target some distance away from you that is in line with your eye and the pencil. Now, look at the pencil; you should see two targets. Now, look at the target you should see two pencils. The brain takes those two slightly different images, and extracts distance information from them. Interestingly, range finders on some cameras work the same way. You focus by bringing the view of two lenses together. Range finders of this type (with two lenses) are also made for deer hunters, so they can aim their arrows more accurately. Even battleships used retinal disparity to determine range, but the lenses involved were widely separated, and the ranges were measured in miles.
Binocular Convergence
Take your pen or pencil and hold it about 2 feet away from your face. Then, bring it slowly toward your face. At about 7-9 inches away you should feel your eye muscles straining and they will lose focus. Move the pencil or pen within that short range. The muscle strain was caused by your eyes rotating inward to their limit.