The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a wider field of view. For example, a human has a horizontal field of view of approximately 200 degrees with two eyes but only 160 degrees with one. Third, it gives binocular summation in which the ability to detect faint objects is enhanced. Fourth it can give stereopsis in which parallax provided by the two eyes' different positions on the head give precise depth perception. Such binocular vision is usually accompanied by singleness of vision or binocular fusion, in which a single image is seen despite each eye's having its own image of any object. Other phenomena of binocular vision include utrocular discrimination, eye dominance, allelotropia, and binocular rivalry.

Stereopsis (from stereo meaning solidity, and opsis meaning vision or sight) is the process in visual perception leading to the sensation of depth from the two slightly different projections of the world onto the retinas of the two eyes. The differences in the two retinal images are called horizontal disparity, retinal disparity, or binocular disparity. The differences arise from the eyes' different positions in the head. Stereopsis is commonly referred to as depth perception. This is inaccurate, as depth perception relies on many more monocular cues than stereoptical ones (see Depth perception), and individuals with only one functional eye still have full depth perception except in artificial cases (such as stereoscopic images) where only binocular cues are present.

Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. Examples include rabbits, buffalos, 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.

Other animals, usually predatory animals, have their two eyes positioned on the front of their heads, thereby reducing field of view in favour of stereopsis. Examples include eagles, cats, and snakes.

Some predator animals, such as sperm whales, have their two eyes positioned on opposite sides of their heads. Other animals that are not necessarily predators, such as fruit bats and some primates also have forward facing eyes. These are usually animals that need fine depth discrimination, for example, to pick fruit or to find a branch.

In animals with forward-facing eyes, the eyes usually move together.

When the eyes move laterally, in the same direction, this is called a version. When the eyes move in opposite directions, to an object closer than where the eyes are pointing or farther than where the eyes are pointing, this is called a vergence. When the eyes move in, it is a convergence eye movement; when the eyes move out, it is a divergence eye movement.

Some animals (including some humans, notably exotropes) 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 to be 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.

For more information and pictures:
Binocular Vision
Stereopsis

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