What is stereopsis?

Stereopsis (from the Greek stereoV, meaning solid) is the exploitation of binocular vision (i.e., the fact that we have two eyes with overlapping visual fields) for the purposes of three-dimensional (3D) vision.

Because our two eyes usually look the same general direction, but from slightly different positions (an adult’s eyes are displaced horizontally by about 6 cm), they see slightly different views of the world. Consider what happens when you look at your computer monitor, which is probably about 18 inches (about 46 cm) away. The image of the point where you’re looking falls on corresponding parts of the your retinas. Specifically, the image falls on each eye’s fovea, which is the part of your retina with the best resolution. However, what if your finger is looming directly between you and where you’re looking on the monitor? It will be slightly to the right of where your left eye is pointed and slightly to the left of where your right eye is pointed. Therefore, the image of your finger will fall on slightly noncorresponding parts of your retinas. It is this geometry that your brain exploits in generating a 3D percept: the extent to which an object’s images falls on noncorresponding parts of your retinas depends on how far away it is. Therefore, by noticing the extent to which different images fall on noncorresponding parts of your retinas, the brain can figure out how far away an object is.

I’ve got a nice diagram of all this, but it’s in a format that my software at home can’t read. Some day when I’m at work and can steal the time, I’ll put up the diagram and elaborate my explanation.