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What is Stereopsis?

Stereopsis (also called Binocular Depth Perception or 3-D vision) is an important aspect of normal, healthy vision. The result of good stereoscopic vision is

  • the ability to visually perceive depth and three dimensional space
  • the ability to visually judge relative distances between objects

It is a perceptual skill that aids accurate movement in three-dimensional space.

How does it work?

Human beings generally come equipped with two eyes and one head. Unlike e.g. horses, humans have two eyes located side-by-side in the front of their heads. Thanks to the close side-by-side positioning, each eye takes a view of the same area from a slightly different angle. The two eye views have plenty in common, but each eye picks up visual information the other does not.

Each eye captures its own view and the two separate images are sent on to the brain for processing. When the two images arrive simultaneously in the back of the brain, they are united into one picture. The mind combines the two images by matching up the similarities and adding in the small differences. The small differences between the two images add up to a big difference in the final picture! The combined image is more than the sum of its parts. It is a three-dimensional stereo picture. It is the added perception of the depth dimension that makes stereo vision so rich and special.

With stereo vision, we can see WHERE objects are in relation to our own bodies with much greater precision - especially when those objects are moving toward or away from us in the depth dimension. We can see a little bit around solid objects without moving our heads and we can even perceive and measure "empty" space with our eyes and brains.

Monocular depth perception requires cues like shading, interposition and apparent size difference.

You need stereopsis because . . .

  • Many occupations are not open to people who have good vision in one eye only - that means no binocular depth perception.
  • Some occupations that depend on stereo vision are: cricket player, waitress, driver, architect, surgeon, dentist.

Here are just a few examples of general actions that depend heavily on stereo vision:

  • Throwing, catching or hitting a ball
  • Driving and parking a car
  • Planning and building a three-dimensional object
  • Threading a needle and sewing
  • Reaching out to shake someone's hand
  • Pouring into a container
  • Stepping off a curb or step

How do you know you have stereo vision?

It is hard to know what you're missing, if you've never had it. Do you see with both your eyes? Are your two eyes similar or different in sight? If the two eye views are too different and cannot be matched up, the brain is forced to make a choice. It will reject all or part of the information from one eye. The brain can suppress or turn off visual information it cannot use.

Conditions that can cause a difference in the eyes or prevent them from working together include:

  • partial or no vision in an eye due to eye injury
  • big difference in short- or farsightedness (known as high anisometropia)
  • squint - intermittent or permanent
  • amblyopia (lazy eye) - often caused by anisometropia and / or squint

If you notice a significant difference in your eyes or have trouble seeing 3-D, a comprehensive eye examination which includes the testing of binocular vision is necessary.