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Basic mathematical/logical reasoning makes it clear that you need two eyes to have depth of vision. By seeing an object from two perspectives, our brain can calculate the distance of the object based on the angle from each eye to the object. However even when I close one eye, I am able to judge reasonably well the depth of objects. Furthermore when I look at a picture I can instantly (I tried to be as unbiased as I could about this) tell it was a picture. How are we able to do this?
Depth perception consists of what are called monocular cues and binocular cues. As you mention, binocular vision has a lot of advantages for depth perception, but it is not completely necessary.
Many animals, particularly those that don't need especially precise vision, have little to no binocular vision, opting instead for visual coverage in more directions by having eyes on the side of the head.
The Wikipedia page I linked at the beginning has a whole list of various monocular cues, but I will mention a few of them here as well that I think roughly outline the different "categories" of monocular cues.
Relative size is a simple one: you expect certain objects to be a certain size. You can use that information to approximate distance.
Motion parallax: moving relative to objects gives you more information. Nearby objects move very quickly across the eye when you move, far away objects do not.
Accommodation: when you focus on a nearby object, you can estimate the depth by how much the lens needs to be deformed to bring the object in focus. The brain gets a copy of that muscle signal that it can use to estimate distance for nearby objects.
Although monocular depth perception is actually quite good out in the natural world, it is possible to trick the system with sufficiently clever strategies, like many other types of optical illusions. The Ames room is one of these.
Monocular Cues for Depth Perception
One way that we perceive depth in the world around us is through the use of what are known as monocular cues. These are clues that can be used for depth perception that involve using only one eye. If you try closing one eye, it might be more difficult to judge depth, but you're still able to detect how near or far objects are in relation to your position.
Depth perception allows us to perceive the world around us in three dimensions and to gauge the distance of objects from ourselves and from other objects. You can contrast monocular cues with binocular cues, which are those that require the use of both eyes.
These are some of the common monocular cues that we use to help perceive depth.
Chameleon eyes feature a negative lens, meaning that the lens is concave. This increases retinal image size, allowing more precise focusing.   In fact, image magnification in chameleons is higher in a scaled comparison to all other vertebrates eyes. 
While the lens are negative, the cornea of chameleon eyes are positive, meaning that it is convex. The increased power of the cornea also contributes to more precise focusing than in other vertebrates.  The cornea improves sight resolution in a narrower field of vision. 
The combination of a negative lens and a positive cornea in the chameleon eye allow for accurate focusing by corneal accommodation.  Using corneal accommodation for depth perception  makes the chameleon the only vertebrate to focus monocularly.  While sight is primarily independent in the two chameleon eyes, the eye that first detects prey will guide accommodation in the other eye.  Contrary to the previous belief that chameleons used stereopsis (both eyes) for depth perception, research has shown monocular focusing to be more likely.  Depending on the chameleon's step in the predation sequence, corneal accommodation can be coupled, meaning the eyes independently focus on the same object.  When scanning the environment and in judging distance to prey, vision and accommodation are uncoupled: the eyes are focusing on different objects, such as the environment and the newly-sighted prey. Immediately before the chameleon's characteristic tongue is extended, accommodation in both eyes is coupled: both eyes focus independently on the prey.  Imprecise alignment of the images from each eye, as demonstrated by measuring various angles from eye to target, shows that stereopsis is unlikely for depth perception the chameleon. 
The nodal point in the eye is the point at which "lines connecting points in the scene and corresponding points in the image intersect."  In chameleons, the nodal point is located a significant distance before the center of rotation, the point around which the eye rotates in the eye socket. As a result of this nodal point separation, images of objects move more or less on the retina based on their distance from the chameleon. The position of an image on the retina is the "primary means by which chameleons judge distance."  Therefore, the rotation of one eye informs the chameleon of the "relative distances of different objects."  An important effect of the ability to judge distance with one eye is that the head does not have to be turned to allow stereoptic viewing of the object. 
Chameleons as an evolutionary transition to stereopsis Edit
A suggested theory for the evolution of squamate vision is that corneal accommodation and monocular depth perception are "primitive" mechanisms in comparison to binocular vision and stereopsis.  Chameleons use an alternative strategy to stereopsis in functional coupling of the eyes immediately before the tongue shot. This differs from stereopsis in that the images from both eyes are not reconciled into one. However, it is possible that this was first used for neural static reduction.  This suggests that chameleons could be seen as a transition between independent and coupled eye use.  However, it is also possible that the chameleon vision system is an alternative, equally successful mode of prey capture and predator avoidance, and perhaps more appropriate for the chameleon's niche as a camouflaged, arboreal hunter than other vision systems.
Prey/predator causes of chameleon eye development Edit
The chameleon, a camouflaged, slow-moving lizard, is an arboreal hunter that hides and ambushes prey.  Prey and predators alike can be sighted and monitored using monocular depth perception. Also, nodal point separation allows distance to be judged with one eye, so minimal head movement is needed by the chameleon in watching its surroundings, reinforcing the chameleon strategy of inconspicuousness. 
Prey capture Edit
The specialized strategy by which chameleons capture prey is reflected in their sensory anatomy and use, in particular that of vision.  First, prey is sighted and distance assessed using one eye.  To avoid detection by prey, a chameleon uses minimal head movement, made possible by nodal point separation.  The chameleon then slowly turns its head toward the prey. Both eyes focus independently on the prey before the tongue shot. 
Predator avoidance Edit
The chameleon predator avoidance response is vision-mediated.  In predator avoidance, chameleons use minimal head movement and a unique method to monitor potential threats. Due to nodal point separation, a chameleon can judge distance to a potential threat with minimal head movement needed. When confronted with a potential threat, chameleons rotate their slender bodies to the opposite side of their perch to avoid detection.  They will keep moving around the branch to keep the branch between themselves and the threat and to keep the threat in their line of sight.  If the branch is narrow, a chameleon can observe a threat binocularly around the branch. While a wide branch might present a difficulty in depth perception to another lizard as it is forced to view the threat monocularly, a chameleon due to corneal accommodation and nodal point separation can judge distance between itself and a potential threat with only one eye viewing the threat. 
Comparison to the sandlance fish Edit
While the chameleon eye is unique in lizards, parallels exist in other animals. In particular, the barred sandburrower fish shares key vision features with the chameleon. This is because the environmental circumstances such as the need for camouflaged quick prey capture that led to the development of the chameleon eye seem to have acted on the sandburrower fish as well.  Rapid predatory attacks are made possible through the chameleon and the sandlances' striated corneal muscles allowing for corneal accommodation, a reduced power lens, and increased corneal power.  A nearly complete eyelid covering and lack of head movement due to nodal separation reduce conspicuousness to prey and predators. 
The term binocular comes from two Latin roots, bini for double, and oculus for eye. 
Some animals - usually, but not always, prey animals - have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. Examples include rabbits, buffaloes, and antelopes. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view.
Some other animals - usually, but not always, predatory animals - have their two eyes positioned on the front of their heads, thereby allowing for binocular vision and reducing their field of view in favor of stereopsis. However, front-facing eyes are a highly evolved trait in vertebrates, and there are only three extant groups of vertebrates with truly forward-facing eyes: primates, carnivorous mammals, and birds of prey.
Some predator animals, particularly large ones such as sperm whales and killer whales, have their two eyes positioned on opposite sides of their heads, although it is possible they have some binocular visual field.  Other animals that are not necessarily predators, such as fruit bats and a number of primates also have forward-facing eyes. These are usually animals that need fine depth discrimination/perception for instance, binocular vision improves the ability to pick a chosen fruit or to find and grasp a particular branch.
The direction of a point relative to the head (the angle between the straight ahead position and the apparent position of the point, from the egocenter) is called visual direction, or version. The angle between the line of sight of the two eyes when fixating a point is called the absolute disparity, binocular parallax, or vergence demand (usually just vergence). The relation between the position of the two eyes, version and vergence is described by Hering's law of visual direction.
In animals with forward-facing eyes, the eyes usually move together.
Eye movements are either conjunctive (in the same direction), version eye movements, usually described by their type: saccades or smooth pursuit (also nystagmus and vestibulo-ocular reflex). Or they are disjunctive (in opposite direction), vergence eye movements. The relation between version and vergence eye movements in humans (and most animals) is described by Hering's law of equal innervation.
Some animals use both of the above strategies. A starling, for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis. A remarkable example is the chameleon, whose eyes appear as if mounted on turrets, each moving independently of the other, up or down, left or right. Nevertheless, the chameleon can bring both of its eyes to bear on a single object when it is hunting, showing vergence and stereopsis.
Binocular summation is the process by which the detection threshold for a stimulus is lower with two eyes than with one.  There are various types of possibilities when comparing binocular performance to monocular.  Neural binocular summation occurs when the binocular response is greater than the probability summation. Probability summation assumes complete independence between the eyes and predicts a ratio ranging between 9-25%. Binocular inhibition occurs when binocular performance is less than monocular performance. This suggests that a weak eye affects a good eye and causes overall combined vision.  Maximum binocular summation occurs when monocular sensitivities are equal. Unequal monocular sensitivities decrease binocular summation. There are unequal sensitivities of vision disorders such as unilateral cataract and amblyopia.  Other factors that can affect binocular summation include are, spatial frequency, stimulated retinal points, and temporal separation. 
Apart from binocular summation, the two eyes can influence each other in at least three ways.
- . Light falling in one eye affects the diameter of the pupils in both eyes. One can easily see this by looking at a friend's eye while he or she closes the other: when the other eye is open, the pupil of the first eye is small when the other eye is closed, the pupil of the first eye is large. and vergence. Accommodation is the state of focus of the eye. If one eye is open and the other closed, and one focuses on something close, the accommodation of the closed eye will become the same as that of the open eye. Moreover, the closed eye will tend to converge to point at the object. Accommodation and convergence are linked by a reflex, so that one evokes the other. . The state of adaptation of one eye can have a small effect on the state of light adaptation of the other. Aftereffects induced through one eye can be measured through the other.
Once the fields of view overlap, there is a potential for confusion between the left and right eye's image of the same object. This can be dealt with in two ways: one image can be suppressed, so that only the other is seen, or the two images can be fused. If two images of a single object are seen, this is known as double vision or diplopia.
Fusion of images (commonly referred to as 'binocular fusion') occurs only in a small volume of visual space around where the eyes are fixating. Running through the fixation point in the horizontal plane is a curved line for which objects there fall on corresponding retinal points in the two eyes. This line is called the empirical horizontal horopter. There is also an empirical vertical horopter, which is effectively tilted away from the eyes above the fixation point and towards the eyes below the fixation point. The horizontal and vertical horopters mark the centre of the volume of singleness of vision. Within this thin, curved volume, objects nearer and farther than the horopters are seen as single. The volume is known as Panum's fusional area (it's presumably called an area because it was measured by Panum only in the horizontal plane). Outside of Panum's fusional area (volume), double vision occurs.
When each eye has its own image of objects, it becomes impossible to align images outside of Panum's fusional area with an image inside the area.  This happens when one has to point to a distant object with one's finger. When one looks at one's fingertip, it is single but there are two images of the distant object. When one looks at the distant object it is single but there are two images of one's fingertip. To point successfully, one of the double images has to take precedence and one be ignored or suppressed (termed "eye dominance"). The eye that can both move faster to the object and stay fixated on it is more likely to be termed as the dominant eye. 
The overlapping of vision occurs due to the position of the eyes on the head (eyes are located on the front of the head, not on the sides). This overlap allows each eye to view objects with a slightly different viewpoint. As a result of this overlap of vision, binocular vision provides depth.  Stereopsis (from stereo- meaning "solid" or "three-dimensional", and opsis meaning “appearance” or “sight”) is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision.  Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due to the eyes' different positions on the head. These differences, referred to as binocular disparity, provide information that the brain can use to calculate depth in the visual scene, providing a major means of depth perception.  There are two aspects of stereopsis: the nature of the stimulus information specifying stereopsis, and the nature of the brain processes responsible for registering that information.  The distance between the two eyes on an adult is almost always 6.5 cm and that is the same distance in shift of an image when viewing with only one eye.  Retinal disparity is the separation between objects as seen by the left eye and the right eye and helps to provide depth perception.  Retinal disparity provides relative depth between two objects, but not exact or absolute depth. The closer objects are to each other, the retinal disparity will be small. If the objects are farther away from each other, then the retinal disparity will be larger. When objects are at equal distances, the two eyes view the objects as the same and there is zero disparity. 
Because the eyes are in different positions on the head, any object away from fixation and off the plane of the horopter has a different visual direction in each eye. Yet when the two monocular images of the object are fused, creating a Cyclopean image, the object has a new visual direction, essentially the average of the two monocular visual directions. This is called allelotropia.  The origin of the new visual direction is a point approximately between the two eyes, the so-called cyclopean eye. The position of the cyclopean eye is not usually exactly centered between the eyes, but tends to be closer to the dominant eye.
When very different images are shown to the same retinal regions of the two eyes, perception settles on one for a few moments, then the other, then the first, and so on, for as long as one cares to look. This alternation of perception between the images of the two eyes is called binocular rivalry.  Humans have limited capacity to process an image fully at one time. That is why the binocular rivalry occurs. Several factors can influence the duration of gaze on one of the two images. These factors include context, increasing of contrast, motion, spatial frequency, and inverted images.  Recent studies have even shown that facial expressions can cause longer attention to a particular image.  When an emotional facial expression is presented to one eye, and a neutral expression is presented to the other eye, the emotional face dominates the neutral face and even causes the neutral face to not been seen. 
To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired. This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of that person with a card. Have the person look at your finger tip. Move the finger around this is to break the reflex that normally holds a covered eye in the correct vergence position. Hold your finger steady and then uncover the person's eye. Look at the uncovered eye. You may see it flick quickly from being wall-eyed or cross-eyed to its correct position. If the uncovered eye moved from out to in, the person has esophoria. If it moved from in to out, the person has exophoria. If the eye did not move at all, the person has orthophoria. Most people have some amount of exophoria or esophoria it is quite normal. If the uncovered eye also moved vertically, the person has hyperphoria (if the eye moved from down to up) or hypophoria (if the eye moved from up to down). Such vertical phorias are quite rare. It is also possible for the covered eye to rotate in its orbit, such a condition is known as cyclophoria. They are rarer than vertical phorias. Cover test may be used to determine direction of deviation in cyclophorias also. 
The cover-uncover test can also be used for more problematic disorders of binocular vision, the tropias. In the cover part of the test, the examiner looks at the first eye as he or she covers the second. If the eye moves from in to out, the person has exotropia. If it moved from out to in, the person has esotropia. People with exotropia or esotropia are wall-eyed or cross-eyed respectively. These are forms of strabismus that can be accompanied by amblyopia. There are numerous definitions of amblyopia.  A definition that incorporates all of these defines amblyopia as a unilateral condition in which vision in worse than 20/20 in the absence of any obvious structural or pathologic anomalies, but with one or more of the following conditions occurring before the age of six: amblyogenic anisometropia, constant unilateral esotropia or exotropia, amblyogenic bilateral isometropia, amblyogenic unilateral or bilateral astigmatism, image degradation.  When the covered eye is the non-amblyopic eye, the amblyopic eye suddenly becomes the person's only means of seeing. The strabismus is revealed by the movement of that eye to fixate on the examiner's finger. There are also vertical tropias (hypertropia and hypotropia) and cyclotropias.
Binocular vision anomalies include: diplopia (double vision), visual confusion (the perception of two different images superimposed onto the same space), suppression (where the brain ignores all or part of one eye's visual field), horror fusionis (an active avoidance of fusion by eye misalignment), and anomalous retinal correspondence (where the brain associates the fovea of one eye with an extrafoveal area of the other eye).
Binocular vision anomalies are among the most common visual disorders. They are usually associated with symptoms such as headaches, asthenopia, eye pain, blurred vision, and occasional diplopia.  About 20% of patients who come to optometry clinics will have binocular vision anomalies.  The most effective way to diagnosis vision anomalies is with the near point of convergence test.  During the NPC test, a target, such as a finger, is brought towards the face until the examiner notices that one eye has turned outward and/or the person has experienced diplopia or doubled vision. 
Up to a certain extent, binocular disparities can be compensated for by adjustments of the visual system. If, however, defects of binocular vision are too great – for example if they would require the visual system to adapt to overly large horizontal, vertical, torsional or aniseikonic deviations – the eyes tend to avoid binocular vision, ultimately causing or worsening a condition of strabismus.
Monocular cues provide depth information when viewing a scene with one eye.
Context-dependent interpretation of the size.
Shots at different distances
The horizon line is at the height of the armrests.
View from a window on the 2nd floor of a house.
Mountain peak near the snow line and several mountain peaks above the snow line.
In spatial vision, the horizontal line of sight can play a role. In the picture taken from the window of a house, the horizontal line of sight is at the level of the second floor (yellow line). Below this line, the further away objects are, the higher up in the visual field they appear. Above the horizontal line of sight, objects that are further away appear lower than those that are more nearby. To represent spatial impressions in graphical perspective, one can use a vanishing point.  When looking at long geographical distances, perspective effects also partially result by the angle of vision, but not only by this. In picture 5 of the series, in the background is Mont Blanc, the highest mountain in the Alps. It appears lower than the mountain in front in the center of the picture. Measurements and calculations can be used to determine the proportion of the curvature of the earth in the subjectively perceived proportions.
Relative size If two objects are known to be the same size (e.g., two trees) but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends the larger visual angle appears closer. Familiar size Since the visual angle of an object projected onto the retina decreases with distance, this information can be combined with previous knowledge of the object's size to determine the absolute depth of the object. For example, people are generally familiar with the size of an average automobile. This prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene. Absolute size Even if the actual size of the object is unknown and there is only one object visible, a smaller object seems further away than a large object that is presented at the same location  Aerial perspective Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance contrast and lower color saturation. Due to this, images seem hazy the farther they are away from a person's point of view. In computer graphics, this is often called "distance fog". The foreground has high contrast the background has low contrast. Objects differing only in their contrast with a background appear to be at different depths.  The color of distant objects are also shifted toward the blue end of the spectrum (e.g., distant mountains). Some painters, notably Cézanne, employ "warm" pigments (red, yellow and orange) to bring features forward towards the viewer, and "cool" ones (blue, violet, and blue-green) to indicate the part of a form that curves away from the picture plane. Accommodation This is an oculomotor cue for depth perception. When we try to focus on far away objects, the ciliary muscles stretch the eye lens, making it thinner, and hence changing the focal length. The kinesthetic sensations of the contracting and relaxing ciliary muscles (intraocular muscles) is sent to the visual cortex where it is used for interpreting distance/depth. Accommodation is only effective for distances greater than 2 meters. Occultation Occultation (also referred to as interposition) happens when near surfaces overlap far surfaces.  If one object partially blocks the view of another object, humans perceive it as closer. However, this information only allows the observer to create a "ranking" of relative nearness. The presence of monocular ambient occlusions consist of the object's texture and geometry. These phenomena are able to reduce the depth perception latency both in natural and artificial stimuli.   Curvilinear perspective At the outer extremes of the visual field, parallel lines become curved, as in a photo taken through a fisheye lens. This effect, although it is usually eliminated from both art and photos by the cropping or framing of a picture, greatly enhances the viewer's sense of being positioned within a real, three-dimensional space. (Classical perspective has no use for this so-called "distortion," although in fact the "distortions" strictly obey optical laws and provide perfectly valid visual information, just as classical perspective does for the part of the field of vision that falls within its frame.) Texture gradient Fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects. Texture gradients are grains of an item. For example, on a long gravel road, the gravel near the observer can be clearly seen of shape, size and colour. In the distance, the road's texture cannot be clearly differentiated. Lighting and shading The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects provide an effective cue for the brain to determine the shape of objects and their position in space.  Defocus blur Selective image blurring is very commonly used in photographic and video for establishing the impression of depth. This can act as a monocular cue even when all other cues are removed. It may contribute to the depth perception in natural retinal images, because the depth of focus of the human eye is limited. In addition, there are several depth estimation algorithms based on defocus and blurring.  Some jumping spiders are known to use image defocus to judge depth.  Elevation When an object is visible relative to the horizon, we tend to perceive objects which are closer to the horizon as being farther away from us, and objects which are farther from the horizon as being closer to us.  In addition, if an object moves from a position close the horizon to a position higher or lower than the horizon, it will appear to move closer to the viewer.
Binocular cues provide depth information when viewing a scene with both eyes.
Stereopsis, or retinal (binocular) disparity, or binocular parallax Animals that have their eyes placed frontally can also use information derived from the different projection of objects onto each retina to judge depth. By using two images of the same scene obtained from slightly different angles, it is possible to triangulate the distance to an object with a high degree of accuracy. Each eye views a slightly different angle of an object seen by the left and right eyes. This happens because of the horizontal separation parallax of the eyes. If an object is far away, the disparity of that image falling on both retinas will be small. If the object is close or near, the disparity will be large. It is stereopsis that tricks people into thinking they perceive depth when viewing Magic Eyes, Autostereograms, 3-D movies, and stereoscopic photos. Convergence This is a binocular oculomotor cue for distance/depth perception. Because of stereopsis the two eyeballs focus on the same object. In doing so they converge. The convergence will stretch the extraocular muscles, the receptors for this are muscle spindles. As happens with the monocular accommodation cue, kinesthetic sensations from these extraocular muscles also help in depth/distance perception. The angle of convergence is smaller when the eye is fixating on far away objects. Convergence is effective for distances less than 10 meters.  Shadow Stereopsis Antonio Medina Puerta demonstrated that retinal images with no parallax disparity but with different shadows are fused stereoscopically, imparting depth perception to the imaged scene. He named the phenomenon "shadow stereopsis". Shadows are therefore an important, stereoscopic cue for depth perception. 
Of these various cues, only convergence, accommodation and familiar size provide absolute distance information. All other cues are relative (i.e., they can only be used to tell which objects are closer relative to others). Stereopsis is merely relative because a greater or lesser disparity for nearby objects could either mean that those objects differ more or less substantially in relative depth or that the foveated object is nearer or further away (the further away a scene is, the smaller is the retinal disparity indicating the same depth difference.)
The law of Newton-Müller-Gudden Edit
Isaac Newton proposed that the optic nerve of humans and other primates, has a specific architecture on its way from the eye to the brain. Nearly half of the fibres from the human retina project to the brain hemisphere on the same side as the eye from which they originate. That architecture is labelled hemi-decussation or ipsilateral (same sided) visual projections (IVP). In most other animals these nerve fibres cross to the opposite side of the brain.
Bernhard von Gudden showed that the OC contains both crossed and uncrossed retinal fibers, and Ramon y Cajal  observed that the grade of hemidecussation differs between species   Walls  [ page needed ] formalized a commonly accepted notion into the law of Newton-Müller-Gudden (NGM) saying: that the degree of optic fibre decussation in the optic chiasm is contrariwise related to the degree of frontal orientation of the optical axes of the eyes. In other words that the number of fibers that do not cross the midline is proportional to the size of the binocular visual field. However, an issue of the Newton-Müller-Gudden law is the considerable interspecific variation in IVP seen in non-mammalian species. That variation is unrelated to mode of life, taxonomic situation, and the overlap of visual fields. 
Thus, the general hypothesis was for long that the arrangement of nerve fibres in the optic chiasm in primates and humans has developed primarily to create accurate depth perception, stereopsis, or explicitly that the eyes observe an object from somewhat dissimilar angles and that this difference in angle assists the brain to evaluate the distance.
The Eye-forelimb EF hypothesis Edit
The EF hypothesis suggests that the need of accurate eye hand control was key in the evolution of stereopsis. According to the EF hypothesis, stereopsis is evolutionary spinoff from a more vital process: that the construction of the optic chiasm and the position of eyes (the degree of lateral or frontal direction) is shaped by evolution to help the animal to coordinate the limbs (hands, claws, wings or fins). 
The EF hypothesis postulates that it has selective value to have short neural pathways between areas of the brain that receive visual information about the hand and the motor nuclei that control the coordination of the hand. The essence of the EF hypothesis is that evolutionary transformation in OC will affect the length and thereby speed of these neural pathways.  Having the primate type of OC means that motor neurons controlling/executing let us say right hand movement, neurons receiving sensory e.g. tactile information about the right hand, and neurons obtaining visual information about the right hand, all will be situated in the same (left) brain hemisphere. The reverse is true for the left hand, the processing of visual, tactile information, and motor command – all of that takes place in the right hemisphere. Cats and arboreal (tree-climbing) marsupials have analogous arrangements (between 30 to 45% of IVP and forward directed eyes). The result will be that visual info of their forelimbs reaches the proper (executing) hemisphere. The evolution has resulted in small, and gradual fluctuations to the direction of the nerve pathways in the OC. This transformation can go in either direction.   Snakes, cyclostomes and other animals that lack extremities have relatively many IVP. Notably these animals have no limbs (hands, paws, fins or wings) to direct. Besides, left and right body parts of snakelike animals cannot move independently of each other. For example if a snake coils clockwise, its left eye only sees the left body-part and in anti-clock-wise position the same eye will see just the right body-part. For that reason, it is functional for snakes to have some IVP in the OC (Naked). Cyclostome descendants (in other words most vertebrates) that due to evolution ceased to curl and, instead developed forelimbs would be favored by achieving completely crossed pathways as long as forelimbs were primarily occupied in lateral direction. Reptiles such as snakes that lost their limbs, would gain by recollect a cluster of uncrossed fibres in their evolution. That seems to have happened, providing further support for the EF hypothesis.  
Mice’ paws are usually busy only in the lateral visual fields. So, it is in accordance with the EF hypothesis that mice have laterally situated eyes and very few crossings in the OC. The list from the animal kingdom supporting the EF hypothesis is long (BBE). The EF hypothesis applies to essentially all vertebrates while the NGM law and stereopsis hypothesis largely applies just in mammals. Even some mammals display important exceptions, e.g. dolphins have only uncrossed pathways although they are predators. 
It is a common suggestion that predatory animals generally have frontally-placed eyes since that permit them to evaluate the distance to prey, whereas preyed-upon animals have eyes in a lateral position, since that permit them to scan and detect the enemy in time. However, many predatory animals may also become prey, and several predators, for instance the crocodile, have laterally situated eyes and no IVP at all. That OC architecture will provide short nerve connections and optimal eye control of the crocodile's front foot. 
Birds, usually have laterally situated eyes, in spite of that they manage to fly through e.g. a dense wood. In conclusion, the EF hypothesis does not reject a significant role of stereopsis, but proposes that primates' superb depth perception (stereopsis) evolved to be in service of the hand that the particular architecture of the primate visual system largely evolved to establish rapid neural pathways between neurons involved in hand coordination, assisting the hand in gripping the correct branch 
Most open-plains herbivores, especially hoofed grazers, lack binocular vision because they have their eyes on the sides of the head, providing a panoramic, almost 360°, view of the horizon - enabling them to notice the approach of predators from almost any direction. However, most predators have both eyes looking forwards, allowing binocular depth perception and helping them to judge distances when they pounce or swoop down onto their prey. Animals that spend a lot of time in trees take advantage of binocular vision in order to accurately judge distances when rapidly moving from branch to branch.
Matt Cartmill, a physical anthropologist & anatomist at Boston University, has criticized this theory, citing other arboreal species which lack binocular vision, such as squirrels and certain birds. Instead, he proposes a "Visual Predation Hypothesis," which argues that ancestral primates were insectivorous predators resembling tarsiers, subject to the same selection pressure for frontal vision as other predatory species. He also uses this hypothesis to account for the specialization of primate hands, which he suggests became adapted for grasping prey, somewhat like the way raptors employ their talons.
Photographs capturing perspective are two-dimensional images that often illustrate the illusion of depth. Photography utilizes size, environmental context, lighting, textural gradience, and other effects to capture the illusion of depth.  Stereoscopes and Viewmasters, as well as 3D films, employ binocular vision by forcing the viewer to see two images created from slightly different positions (points of view). Charles Wheatstone was the first to discuss depth perception being a cue of binocular disparity. He invented the stereoscope, which is an instrument with two eyepieces that displays two photographs of the same location/scene taken at relatively different angles. When observed, separately by each eye, the pairs of images induced a clear sense of depth.  By contrast, a telephoto lens—used in televised sports, for example, to zero in on members of a stadium audience—has the opposite effect. The viewer sees the size and detail of the scene as if it were close enough to touch, but the camera's perspective is still derived from its actual position a hundred meters away, so background faces and objects appear about the same size as those in the foreground.
Trained artists are keenly aware of the various methods for indicating spatial depth (color shading, distance fog, perspective and relative size), and take advantage of them to make their works appear "real". The viewer feels it would be possible to reach in and grab the nose of a Rembrandt portrait or an apple in a Cézanne still life—or step inside a landscape and walk around among its trees and rocks.
Cubism was based on the idea of incorporating multiple points of view in a painted image, as if to simulate the visual experience of being physically in the presence of the subject, and seeing it from different angles. The radical experiments of Georges Braque, Pablo Picasso, Jean Metzinger's Nu à la cheminée,  Albert Gleizes's La Femme aux Phlox,   or Robert Delaunay's views of the Eiffel Tower,   employ the explosive angularity of Cubism to exaggerate the traditional illusion of three-dimensional space. The subtle use of multiple points of view can be found in the pioneering late work of Cézanne, which both anticipated and inspired the first actual Cubists. Cézanne's landscapes and still lives powerfully suggest the artist's own highly developed depth perception. At the same time, like the other Post-Impressionists, Cézanne had learned from Japanese art the significance of respecting the flat (two-dimensional) rectangle of the picture itself Hokusai and Hiroshige ignored or even reversed linear perspective and thereby remind the viewer that a picture can only be "true" when it acknowledges the truth of its own flat surface. By contrast, European "academic" painting was devoted to a sort of Big Lie that the surface of the canvas is only an enchanted doorway to a "real" scene unfolding beyond, and that the artist's main task is to distract the viewer from any disenchanting awareness of the presence of the painted canvas. Cubism, and indeed most of modern art is an attempt to confront, if not resolve, the paradox of suggesting spatial depth on a flat surface, and explore that inherent contradiction through innovative ways of seeing, as well as new methods of drawing and painting.
In robotics and computer vision, depth perception is often achieved using sensors such as RGBD cameras. 
Cataract Surgery Options for Distance, Intermediate and Near Vision
There are many options to consider once you’ve made the decision to have your cataract removed. Cataract is the name given to the natural lens of the eye when it becomes cloudy or opaque. The natural lens is the main focusing element of the eye. If it is removed without replacing it with another lens, the vision would be very poor. That is why cataract surgery involves both the removal of the cataract and the placement of an intraocular lens (IOL).
The natural lens of the eye can change shape when we are younger, allowing us to focus on near objects when accommodating and to see in the distance when the eye is relaxed. Over time and with age, the eye and lens slowly loses it’s ability to focus on near objects. This process is called presbyopia, and explains why many people require reading glasses as they reach their early to mid forties. When a cataract is removed and an IOL is placed, the eye will not have the same ability to accommodate to see near objects, as the IOLs do not change shape like the natural lens does.
It is important to understand that there are choices available when it comes to deciding which IOL to place. There are monofocal IOLS, which have one focal point, and there are multifocal IOLs which have been designed to have two or three focal points. Multifocal lenses are designed to have different areas of the lens with different focal points to allow you to see in the distance as well as near. However, there are important limitations in their ability to understand before deciding on a multifocal lens. The easiest way to describe this limitation is with an example. If you consider all of the light entering the eye to be 100% of the light, then an multifocal lens may focus 50% of that light at distance and 50% of light at near. That means that it is not possible for 100% of the light to be focused all at distance or all at near, as there is always some fraction of the total focused at both distances. Patients are more likely to complain after having a multifocal lens placed that their distance vision and near vision is not as clear as they had hoped. However, the major benefit of these lenses are that they do provide the ability to see both distance and near without the near for reading glasses. It is particularly important to understand this if there is some other reason for the vision to be compromised, as placing a multifocal lens could lead to further reduced vision.
Monofocal lenses by definition are focused at one distance. It is possible to have both distance and near vision after cataract surgery using a different target focal length for each eye. Most often patients are happier with a distance focal length IOL placed in their dominant eye and a near or intermediate focal length IOL placed in the non-dominant eye. This is termed monovision. An important point to understand is that about 20% of the population will not be able to tolerate monovision, and may double vision after both a distance and near IOL are placed. It is always best to check this prior to surgery.
Please do not hesitate to discuss any of the above issues with me at the time of your preoperative clinic appointment.
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I have always had visual issues (I'm old now) and I'm quite curious. I was diagnosed with amblyopia as a youth, Identified natural monovision, and stigmatism. I often see double and sometimes, as a child, would seem to see thru items. My eyes were not focusing together and one sees completely different from the other. I had great difficulty reading as I would see the latter part of a sentence before I saw beginning or middle. Transposing was a constant problem. (school stairwell sign always appeared to say" run don't walk"). Glasses would make distant items clear but still too far to read. I usually can read a road sign as I pass it. Ironically, my vision improved a good deal and restrictions were removed from my driver's license, but I really can't see very well. I'm comfortable driving to familiar destinations. Now I'm told cataracts are beginning but not severe enough for surgery. I'd like to believe it will help if I ever get to that point. Would anyone be able to identify a specific problem for my many concerns? anon351201 October 11, 2013
I am 51 years old and I have worn contacts for more than 30 years. Three years ago I began using monovision correction with one contact lens for distance vision, leaving the other eye uncorrected for close vision. (I switch eyes every six months or so.) When I began doing this, my left eye prescription was -3.50 and the right was -3.75. I went without insurance for a couple of years, and just recently went back to the optometrist and my vision in both eyes dramatically improved over the three years since my last exam! I just left the optometrist and my contact lens prescription is -2.25 in both eyes. Has anyone else had dramatic vision improvement as a result of monovision correction? anon312195 January 6, 2013
I had cataract surgery (so far left eye only) with the expectation that I would be corrected for distance and still use readers. It didn't work out that way. I have up close in the left eye and no longer require readers, and, my job requires that I be on a computer all day.
Now it is time for the IOL for the right eye that is my distance eye and I am worried that it may turn out like the left and and up having only near vision acuity.
In short, I am loving the monovision and fear that I may lose it. I guess there is always LASIK for the right eye if I need it.
Just to be clear: throughout my life my left eye was always my dominant eye and that switched in recent years, so shooting and archery were left handed and that is now not an option without corrective lenses. Bummer. anon284176 August 8, 2012
I just had cataract surgery in my left eye. I am nearsighted in my right eye and have a cataract that needs to be surgically removed. I am having a difficult time adjusting to the "monovision" due to this surgery. My optometrist had given me five different prescription strength contact lenses to try to adjust to.
I had previously worn contact lenses in both eyes due to nearsightedness. Any suggestions would be greatly appreciated anon231349 yesterday
I am about eight hours with monovision via contact lenses. I have a reading correction lens in my left eye, and nothing in my right (dominant) eye. I'm loving it! I can read and use the computer without looking for my reading glasses. I am gradually getting used to the difference between each eye. Others state what I have found out: if you think about it, there will be a noticeable difference. If you forget about it, you don't notice. I am delighted with the results. anon206872 August 18, 2011
I've been wearing monovision lenses. These are my regular Acuvues, except my dominant eye has a stronger distance lens and my non-dominant eye has a close correction lens. It's cool how it works. My only issue is the mid-distance like the computer and the grocery store. I'm hoping that gets better, but I can live with it.
I could not live with the bifocal contacts as they gave me really bad headaches even after three weeks. anon138378 December 31, 2010
I am now two hours into monovision with contact lenses. Drove home with them. Testing, reading on my droidx, and watching tv. I have mild eye strain but think this might work! It is so weird that when I remember, things go out of focus and when I forget things are clear. anon120520 October 21, 2010
Monovision is great as long as you aren't expecting perfection. They say presbyopic patients are the most grateful because they can't see clear at any distance. I wore monovision contacts for five years and recently had surgery monovision with lasik.
I'd say my vision is almost exactly as it was with the contacts. Definitely try it with contacts first and give yourself time to adjust. Sometimes when I'm reading, I have tears of joy over the being able to read again without aids. It's really a miracle, Lasik. anon118236 October 13, 2010
I am in my late 50's. I have worn contact lenses since they were introduced and the last couple of years accommodating my vision with readers. This last eye examination, my doctor suggested Monovision. Well, anything is better than readers. Needless to say it's been 2months and I have yet to get used to them. My biggest problem is the feeling of being sleepy. I can focus afar and focus near. Just that sleepy light headed feeling is very annoying.
But like I said anything is better than the readers. I will continue with these lenses and hope that I will someday learn to live with them. anon77250 April 13, 2010
I'm two days into my monos, and my distant vision comes in/out of focus. Near and middle are clear.
First few hours I felt some mild eye strain. Better now.
One silly thing to remember: when you think about it it goes out of focus, as when you forget - it's goes clear! The mind is a funny thing. pretrib October 7, 2009
I was recently told I naturally have monovision. I didn't know what it was until I searched it on the web. I have one eye that sees good up close and the other one sees good far away. The man who gave me the exam was pretty excited about it. He said people wear contacts or have surgery to have this condition. I would have never known. anon35372 July 4, 2009
I am seriously considering monovision lenses as using reading glasses. where are they? did I leave them at home? etc etc is very annoying and titing. I am looking for input on this as I try to consider this option. finwood December 18, 2008
I have just started with monovision. (about six hours) I can read now without reading glasses, but I am having some problems with far away. My question is . How long does it take the brain to adjust to this. anon1272 yesterday
I need Customvue PRK on my non-dominant left eye to correct for blurred/mild ghost images but am considering leaving it slightly under corrected for reading without glasses as I age. My work requires reading fine print from a distance of 1-3 feet away and I desperately want to delay the need for reading glasses/contacts as long as possible. I'm 39 years old with -0.5 nearsighted vision with 2 diopeters of irregular astignmatism and thin cornea (430) in my left non-dominant eye. Nine months ago I had Customvue PRK done on my dominant right eye with similar specs and quality results - 20/15 far vision and solid clear near vision for reading fine print as close (but not closer than) one foot away.
Am I right in thinking that less ablation and reading fine print in the one to three feet range should be better preserved if I leave my left eye slightly nearsighted (now it reads fine print clearly only from 6-10 inches away) and correct primarily for astigmatism and higher order aberrations? Or does 20/15 vision in my right eye substantially delay presbyopia's effect as I age in my critical reading distance (1-3 feet) and negate the need to under correct my left eye?
Importance of Depth Perception
Depth perception is important to our everyday life in so many ways. It allows us to move through life without bumping into things. Without it, you wouldn’t know how far away a wall was from you or the distance from your car to the car in front of you.
It also lets you determine how fast an object is coming towards you. This skill is important if you are crossing the street and there are cars coming or if you want to pass a slow car and have to go into the oncoming traffic lane to do so. Depth perception keeps you safe in these types of situations. If your depth perception is off, you likely will not be able to drive safely.
So how do you know if your depth perception isn’t functioning properly? If you have troubling judging how quickly an object is coming towards you, such as a car or even a ball that is being rolled to you, you may have poor depth perception. For a true diagnosis for this important field of vision, visit a qualified optician. They can administer a test with the Howard-Dolman apparatus. This can help narrow down the cause of your problem so that you may address the problem directly. Visiting an optometrist regularly is essential to preserving your vision, so do not neglect to schedule your appointments.
Vision Changes Not Associated With Stroke
There are several common vision problems that are caused by eye problems, heredity or other diseases, but not a stroke.
- Floaters: If you see occasional "floating" spots, this is usually a sign of aging, or sometimes a sign of diabetic eye disease, which may cause more serious vision changes if left untreated. If floaters persist, you should get medical attention to prevent further complications.
- Seeing Halos Around Lights: A cataract, which is often the result of normal aging, diabetes or smoking, causes a sense that you are looking through a cloudy or frosty glass. Cataracts can be effectively and safely treated.
- Near-Sighted or Far-Sighted: The common problems of nearsightedness and farsightedness are a result of imperfect focusing of the eyes. People who are nearsighted have difficulty focusing on faraway objects, while people who are farsighted have difficulty focusing on close objects. These are hereditary problems or a result of normal aging, but not a stroke.
- Triple Vision: There is no real biological reason for triple vision. People who claim to see multiple objects may be under the influence of medications or drugs or experiencing a psychiatric issue.
- Red Green Color Blindness: Red-green color blindness is different from achromatopsia (when a person can't see color.) Red-green color blindness is a hereditary condition caused by a genetic defect.
A Word From Verywell
One of our most important senses is the sense of vision. Vision requires a complex interaction between the eyes and the brain. A stroke can cause several different changes in vision, depending on the size of the stroke, and which region of the brain is affected. Rehabilitation for vision loss is a long process that requires a great deal of patience and persistence.
Depth Perception vs. Binocular Vision
Take a second and try this, close an eye and then stick out your index fingers and point them at each other. Start about six inches apart and bring them together quickly until the touch. Did you line up perfectly? Chances are you missed by a little. Now try it with both eyes open.
It is much easier to have your two fingers align squarely with both eye open.This is because you have binocular vision it provides us the ability to discriminate small changes in distance when using two eyes. It goes away when we close an eye. It works using the idea of “normal” double vision.
Now I want you to look out your window off in the distance, there is probably a car and a building. Which one is closer too you? Now close an eye, is it more difficult to tell?
At distances greater than arms length we really do not use binocular vision. We can judge depth with one eye or both eyes equally.
Depth perception means the ability to determine what is closer to us, but the tools we use to do this vary. Up close the most important one is binocular vision. At distance binocular vision really is not useful, we use other tools there including shadowing, lighting, and obstruction (meaning if a car blocks our view of a house, we know the car is closer).
Binocular vision requires two well aligned, well seeing eyes. Therefore individuals with eyeturns (strabismus) or large amount of amblyopia in one eye will not develop binocular vision (without treatment). However these individuals will still have depth perception but up close they may find it more difficult to thread a needle or cap a pen.The other common drawback is they will not appreciate 3-D movies such as Avatar or Alice in Wonderland.
To sum up, if you lack binocular vision it does not mean you don’t have depth perception. You can still judge depth, but judging small distances up close will be more difficult.
How the Eye Works . The eye works like a camera. Light rays enter it through the adjustable iris and are focused by the lens onto the retina, a thin light-sensitive layer which corresponds to the film of the camera. The retina converts the light rays into nerve impulses, which are relayed to the visual center. There the brain interprets them as images.
Like a camera lens, the lens of the eye reverses images as it focuses them. The images on the retina are upside down and they are &ldquoflipped over&rdquo in the visual center. In a psychology experiment, a number of volunteers wore glasses that inverted everything. After 8 days, their visual centers adjusted to this new situation, and when they took off the glasses, the world looked upside down until their brain centers readjusted.
The retina is made up of millions of tiny nerve cells that contain specialized chemicals that are sensitive to light. There are two varieties of these nerve cells, rods and cones . Between them they cover the full range of the eye's adaptation to light. The cones are sensitive in bright light, and the rods in dim light. At twilight, as the light fades, the cones stop operating and the rods go into action. The momentary blindness experienced on going from bright to dim light, or from dim to bright, is the pause needed for the other set of nerve cells to take over.
The rods are spread toward the edges of the retina, so that vision in dim light is general but not very sharp or clear. The cones are clustered thickly in the center of the retina, in the fovea centralis. When the eyes are turned and focused on the object to be seen the image is brought to the central area of the retina. In very dim light, on the other hand, an object is seen more clearly if it is not looked at directly, because then its image falls on an area where the rods are thicker.
Color Vision . Color vision is a function of the cones. The most widely accepted theory is that there are three types of cones, each containing chemicals that respond to one of the three primary colors (red, green, and violet). White light stimulates all three sets of cones any other color stimulates only one or two sets. The brain can then interpret the impulses from these cones as various colors. Man's color vision is amazingly delicate a trained expert can distinguish among as many as 300,000 different hues.
Color vision deficiency (popularly called &ldquocolor blindness&rdquo) is the result of a disorder of one or more sets of cones. The great majority of people with some degree of deficiency lack either red or green cones, and cannot distinguish between those two colors. Complete color vision deficiency (monochromatic vision ), in which none of the sets of color cones works, is very rare. Most deficiencies of color vision are inherited, usually by male children through their mothers from a grandfather with the condition.
Stereoscopic Vision . Stereoscopic vision, or vision in depth, is caused by the way the eyes are placed. Each eye has a slightly different field of vision. The two images are superimposed on one another, but because of the distance between the eyes, the image from each eye goes slightly around its side of the object. From the differences between the images and from other indicators such as the position of the eye muscles when the eyes are focused on the object, the brain can determine the distance of the object.
Stereoscopic vision works best on nearby objects. As the distance increases, the difference between the left-eyed and the right-eyed views becomes less, and the brain must depend on other factors to determine distance. Among these are the relative size of the object, its color and clearness, and the receding lines of perspective. These factors may fool the eye for example, in clear mountain air distant objects may seem to be very close. This is because their sharpness and color are not dulled by the atmosphere as much as they would be in more familiar settings.
Patient Care. Visually handicapped persons who are visiting a clinic for the first time or being admitted to a hospital room require orientation to their environment. Ambulatory patients can be walked around to familiarize them with the location of the bathroom and any other facility they may need to use.
Patients who are in bed following surgery or for therapeutic rest should have articles on their bedside table arranged in the same way all of the time so that they can be found easily. If only one eye is affected, articles should be placed within reach on the unaffected side and persons communicating with the patient also should stand on that side. If peripheral vision is limited, objects and persons should be positioned in the patient's line of vision.
Some patients, especially the elderly, may experience increased sensitivity to glare. Wearing sunglasses outdoors, adjusting the window blinds to deflect the sun, and using indirect lighting can help avoid discomfort. This does not mean that the patient should be in a darkened room. For most, increased illumination makes it easier to see. It is the glare that impairs their vision.
Whenever it is necessary to do something for the visually impaired person, explain beforehand what will be done. This helps reduce confusion and establishes trust in the caregiver. (For patient care, see also blindness .)
Patients with impaired vision may also benefit from such low-vision aids as convex or magnifying lenses that are hand held or mounted on a stand or clipped to the eyeglasses. Adjustable lamps, large-print reading matter, reading stands, writing guides and lined paper, and felt-tipped pens can facilitate reading and writing and improve the quality of life of a person with limited vision.
Categories of nursing diagnoses associated with impaired vision include Anxiety, Ineffective Coping Patterns, Fear of Total Blindness, Impaired Home Maintenance Management, Potential for Physical Injury, Impaired Physical Mobility, Self-Care Deficit, and Self-Imposed Social Isolation.