Ability to determine visually the distance between objects.
We can determine the relative distance of objects in two different ways. One uses cues involving only one eye; the second requires two eyes. When something is far from us, we rely on monocular cues, those that require the use of only one eye. For closer objects, we use both monocular cues and binocular cues, those that necessitate both eyes.
The ability to perceive depth seems to exist early in life. Research with infants has revealed that by two months of age, babies can perceive depth. Prior to that, they may be unable to do so in part because of weak eye muscles that do not let them use binocular depth cues.
Monocular Depth Cues. Psychologists have identified two different kinds of monocular cues. One comes into play when we use the muscles of the eye to change the shape of the eye's lens to focus on an object. We make use of the amount of muscular tension to give feedback about distance.
A second kind of monocular cue relates to external visual stimuli. These cues appear in the table below. Artists use these visual cues to make two dimensional paintings appear realistic. These cues may seem obvious to us now, but artistic renderings from earlier than about the sixteenth century often seem distorted because artists had not yet developed all the techniques to capture these visual cues.
Binocular Cues. Binocular cues require that we use both eyes. One cue makes use of the fact that when we look at a nearby object with both eyes, we bring our eyes together; the muscle tension associated with looking at close objects gives us information about their distance. The second binocular cue involves retinal disparity. This means that each eye (or, more specifically, the retina of each eye) has a slightly different perspective. The slight difference in appearance of an object in each eye when we gaze at it gives us further information about depth. Children's Viewmasters produce a three-dimensional image that has depth because of a slightly different picture that is delivered to each eye. In the natural world, because of the relatively small distance from one pupil to another (about2.5 inches or 6.5 centimeters) binocular cues are effective only for objects that are within about 500 yards (455 m) of the viewer.
Animals that have eyes on front of the face, like primates, will be able to use binocular depth cues because the two eyes see almost, but not quite, the same scene; on the other hand, animals with eyes on the side of the head, like most birds, will be less able to use binocular cues because the visual fields of the two eyes do not overlap very much and each eye sees different scenes.
|MONOCULAR CUE—HOW IT WORKS|
|Aerial Perspective||Objects that are near seem crisper and clearer; far away objects appear fuzzier.|
|Height in Plane||Objects that are farther away appear higher in the visual scene.|
|Interposition||Objects that are nearer block objects that are farther away.|
|Linear Perspective||Lines that are parallel (e.g., railroad tracks) look like they come to a point in the distance. The farther the lines, the closer they are.|
|Motion Parallax||When you are moving and you fixate on a spot, objects closer to you than that spot appear to move in the direction opposite to your motion; objects farther than that spot appear to move in the same direction as you are moving.|
|Relative Size||If two objects are of the same size, the closer one is bigger.|