To illustrate stereoscopic vision, let's do a little test. Stretch out an hand in front of you and with the other one close the right eye first and then the left eye. The result is that the hand seems to shift towards left even if it remained still in its place.

The following picture contains the simulated images that our left and right would report of a same scene; if you look carefully, you will find some little differences. For instance, the italian flag in front of the building is more evident on the left photo.

Left and right views of our eyes

Left-right animation

Our eyes, in fact, record two different images because the distance and angle-shot with which any object is focused are not perfectly the same in both of them.

We see a unique image since our brain, through a process called Stereopsis, fuse them together and exploit the differences returning, with a sort of trigonometric calculation, the third dimension of the space: the depth. Since we are able to determine, even if only roughly, the distance at which an object is placed in the space in relation to our position, we can talk of a three-dimensional or stereoscopic vision; Stereoscopy, in fact, derives from two greek words just to mean Solid Vision.

This phenomenon of the binocular vision was discovered for the first time in 1838 by the english physicist Sir Charles Wheatstone who, furthermore, determined that stereopsis derives from the simultaneous stimulus of retinical elements placed horizontally and that the vertical one doesn't induce any stereoscopic effect: therefore stereoscopic pairs are always to be perfectly allined horizontally. Newbies are warned !

However, it must be said that our brain adoptes also other tricks to evaluate depth besides the binocular vision. In fact, through secondary stereopsis our visual system uses some informations that derive from the relative position of the objects in comparison to others, from prospective, from the shadow and light context, from borders, edges or from our experience with reality.

Useful links:

  Visione Stereoscopica (in italian)
  Virtual Reality : The Sight
  The Analysis of Stereopsis (Poggio)
  The Perception of Depth