Depth perception

Depth perception is the visual ability to perceive the world in three dimensions. It is a trait common to many higher animals. Depth perception allows the beholder to accurately gauge the distance to an object.

Depth clues
Depth perception combines several types of depth clues grouped into two categories: monocular clues (clues available from the input of just one eye) and binocular clues (clues that require input from both eyes).

Monocular clues

 * Motion parallax - The apparent relative motion of several stationary objects against a background when the observer moves gives hints about their relative distance. This effect can be seen clearly when driving in a car, nearby things pass quickly, while far off objects appear stationary. Some animals that lack binocular vision due to wide placement of the eyes employ parallax for depth cueing (e.g. some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines orthogonal to an object of interest to do the same).1
 * Color vision - Correct interpretation of color, and especially lighting cues, allows the beholder to determine the shape of objects.
 * Perspective - The property of parallel lines converging at infinity allows us to reconstruct the relative distance of two parts of an object, or of landscape features.
 * Relative size - An automobile that is close to us seems larger than one that is far away; our visual system exploits the relative size of similar (or familiar) objects to judge distance.
 * Distance fog - Due to light scattering, objects that are a great distance away seem hazier to the eye; the visual system is attuned to this effect.
 * Depth from Focus - The lens of the eye can change its shape to bring objects at different distances into focus. Knowing at what distance the lens is focused when viewing an object means knowing the approximate distance to that object.
 * Occlusion - Occlusion (blocking the sight) of objects by other objects is a clue, albeit a weak one, for judging relative distance. It only allows the beholder to create a "ranking" of nearness, and does not give any insight as to actual distances. In the absence of color vision (as at night) or binocular vision (as with one-eyed creatures) occlusion often serves as the method of last resort for providing rudimentary depth perception.

Binocular clues

 * Stereopsis - By using two images of the same scene taken from slightly different angles, it is possible to triangulate the distance to an object with a high degree of accuracy. Specifically, by fixating on an object in the center of the visual field, the image of that object falls on both retinas. If the object is far away, the distance of that image falling on both retinas will be small.  If the object is close or near, the distance of that image falling on both retinas will be large. This is the major mechanism for depth perception. It is taken advantage of to trick people into thinking they perceive depth when viewing Magic Eyes, Autostereograms, 3D movies and stereoscopic photos.
 * Convergence - The angle of convergence is larger when the eye is fixating on far away objects.

Integrating depth information
Of the above, only convergence, focus and familiar size provide absolute distance information. All other clues are relative (ie, they can only be used to tell which objects are closer relative to others). Integration is carried out automatically by the visual cortex; however, this is largely a process of inference (guesswork) by the brain, and so the beholder may fall victim to optical illusions that result in mistakes in depth perception. However, as J.J. Gibson was keen to point out, there is often sufficient depth information available so as to achieve veridical perception (perceive distance accurately).

Depth perception in art
Photographs capturing perspective are two-dimensional images that often illustrate the illusion of depth. (This differs from a painting, which may use the physical matter of the paint to create a real presence of depth outwardly.)

Trained artists are keenly aware of the above mentioned methods for perceiving depth (color vision, distance fog, perspective and relative sizes). They often take advantage of them to make their works appear real.

Disorders affecting depth perception

 * Ocular conditions such as amblyopia and strabismus may reduce the perception of depth.
 * A person with only one functioning eye has absolutely no binocular depth perception.