Binocular Disparity

Our minds are capable of seeing in three dimensions primarily because we have binocular vision. Binocular vision occurs when two eyes look at the same thing at a slightly different angle, resulting in two slightly different images. It's simple to confirm that as humans we have binocular vision: place your hand a foot in front of your face and alternate closing each eye. Your hand will appear to jump back and forth.

As humans we often take binocular vision for granted. Many animals, such as rabbits and birds, have eyes facing opposite directions. These animals can see all around them without even moving their heads, however do not have two slightly different images of the same thing. For this reason they do not enjoy the many forms of depth perception created by binocular vision.

The slight difference between the viewpoints of your two eyes is called binocular disparity. Binocular disparity is the form of depth perception most used by the human brain, and is the most easily manipulated for perception tricks. The brain takes these two different views and molds them together into a three dimensional object.


	Try it Out: The easiest way to notice Binocular Disparity is to use a periscope. Place a periscope (shorter is better; 3-4 inches will have the best effect) horizontally against one eye, and look at an object. Because the distance between your two "eyes" has now been increased, you will have greater binocular disparity. Objects will appear to have much greater depth differences.

Binocular disparity is only useful when your two eyes see different angles of an image. Look at two parallel horizontal clotheslines, one further away from you than the other. If you only look at the clotheslines and you do not already know how far away each is, binocular disparity will not reveal that the lines are different distances from you. Both eyes see the line as horizontal, so do not give different images to the brain.

Two horizontal bars. It is impossible to tell which is closer.

At an angle the depth difference is easier to see.

There are many ways to create the illusion of depth using binocular disparity. Holograms, stereoscopes, and stereograms all send separate images to each eye. Binocular disparity in the brain combines these images into a three dimensional picture.


	This picture shows how binocular disparity has often been used in comic books, magazines, and movies. The picture contains two images of a chair, one red and one blue, from two slightly different angles. You can "see" the image by putting on special 3D glasses. One lens of the glasses will filter out the blue image, and the other lens will filter out the red image. The result is each of your eyes only receiving one of the two images on the page. Just as if you were looking at a real chair from two different angles, your brain forms these images into one three-dimensional image.

One other experiment to demonstrate binocular disparity is called the Pulfrich Illusion. This illusion occurs because of latency, the fact that the eyes don't react instantaneously to visual stimuli. At lower levels of light, it takes the eye longer to "see an image." If you wear sunglasses on one eye, that one eye will respond more slowly than the uncovered eye.


	The Experiment: Cover one eye with a lens from a pair of sunglasses. This pendulum "swings" back and forth, however your covered eye responds to this swing at a slower speed. This once again results in two different images being sent to the brain, and because of binocular disparity the brain sees the pendulum swinging in a circular path. Try this experiment with a real pendulum for an even better version of this illusion.

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