A creature with two eyes that are used together to view a single image is said to have binocular vision. Creatures with binocular vision have many advantages over creatures with only a single eye or creatures who are not able to use their multiple eyes together. At a very simple level, a second eye provides a safe level of redundancy; if one eye is lost, another is still there, allowing the creature to see. Using two eyes together also improves a creature’s ability to see objects that are, for whatever reason, difficult to see. Generally speaking, because of the distance between two eyes, a creature with binocular vision will have a wider field of vision than a creature without.
Another important aspect of binocular vision is stereopsis, which allows creatures to perceive depth. Stereopsis occurs when two eyes perceive an object and its surroundings from two slightly different angles, which occurs naturally because of the slightly different positions of the eyes. Because of the different angles, the eyes are able to perceive depth and distance. This is especially important to predator animals, who need to judge the distance between themselves and their prey. It is somewhat less important to prey animals; their eyes tend to be very far apart which gives them a much greater field of view.
Having two different eyes that see two slightly different images can have some interesting effects. In some cases, often involving some kind of damage or disorder, one eye sees an image that is quite different from the other eye. In some cases, the different images seen by the eye are fused. Sometimes a creature experiences double vision, in which it sees two often-overlapping images of the object.
There can be many different vision anomalies and disorders that tend to impair vision in different ways. In some people, differences in the muscles of each eye cause one eye or the other to drift, setting proper binocular vision out of alignment. Eye tests searching for binocular vision anomalies usually check to see that an individual’s eyes are both able to work together to follow an object, stay focused on an object, and work together to see only one object at either close or long distances. If the eyes cannot succeed at these tasks, corrective lenses or surgery may be necessary to repair the eyes and restore them to full working order.