Driving Miss Digital

Notes by Dr. Optoglass: Stereoscopy – Stereopsis, Parallax and the Inter-ocular Distance

Topics Covered:

  • Stereopsis
  • Parallax
  • Inter-ocular Distance

You can’t look into a bottle with both eyes at the same time – Togolese Proverb

Stereopsis is the impression of depth we get when we use both eyes, assuming they have normal binocular vision. More than any other depth cue, this gives the greatest impression of space. Why?

At any given moment the brain is getting two streams of information – one from each eye. These images are fused together using a complex mechanism that isn’t very clear yet. The advantage of having two images is the different viewpoints the offer.

The difference in the viewpoints is called parallax. One can measure parallax by measuring the relative distances between objects that seem to move in relation to each other.

The greater the distance between the eyes, the greater the parallax. If you go far enough, the images are so radically different that they cannot be fused together. But we’re lucky. Our eyes are relatively fixed in their sockets, and when focusing at infinity (straight ahead) they are always at a fixed distance from each other.

The distance between the two pupils of each eye, when focusing at infinity, is called the Inter-ocular distance. On average, this distance is approximately 62mm to 65mm, or about 2.5 inches.

Interocular distance

What happens when we are not focusing on infinity? If you focus on your finger and keep bringing it closer, your eyes will almost get cross-eyed. There is one more piece of the stereoscopic puzzle, and that is convergence. We will discuss that next.


  • Stereopsis is the impression of depth we get when we use both eyes.
  • The relative difference between the position of an object when viewed from two different viewpoints is called parallax.
  • The inter-ocular distance of the human eye is about 65mm or 2.5 inches.

Links for further study:

Next: Convergence and Fusion
Previous: Understanding Depth