Why Do We Have Two Eyes?
We have two eyes placed in the front of our heads because we need to be able to judge distances and to see in depth. The change in the position of the eyes from the side of the head in our remote ancestors probably came about because they needed to be able to judge distances accurately as they swung from branch to branch in the trees.
With both eyes in front, their separate fields of vision overlap. We see two images superimposed one upon the other but, because of the space between our eyes; the image from each goes a little way around its own side of the object.
This is called stereoscopic vision, or vision in depth, which we share with apes and monkeys. Many other animals and fish do not have this advantage. To them the world appears flat. One exception is the owl, which sees well than any of us, and has not only stereoscopic vision, but telescopic vision, too.
Our judgment of distances depends, with near objects, upon our stereoscopic vision. As the distance increases, there is less difference between the left-eyed and the right-eyed view. So we depend on other factors as well.
Each eye sees the world from a different angle, creating slightly different pictures. The brain combines these two pictures into a single, three-dimensional image. This is called binocular vision. Seeing in 3-D helps you to judge the distance and size of objects much more easily.
Experience tells us that the farther away an object is, the smaller it looks. Its color also changes, its details disappear, and its outline softens. Nearer objects give us a measure against which to judge the distance of farther ones. Then there is perspective, the familiar illusion that parallel lines converge towards the horizon.