Methods of Depth Perception

   Other than binocular disparity, there are three major methods the brain uses for depth perception: accommodation, convergence, and parallax.

Accommodation

    Accommodation measures distance through the muscles in your eyes. When you look at an object, your eye muscles must adjust the shape of the lens on your eye so that the object is in focus (see Eye Structure).

	Try it Out: Place your hand an inch from your face and try and focus. Feel the strain in your eyes? As you move your hand back away from yourself, your eye muscles slowly relax. The brain determines how tense the muscles are to focus on an object, and uses this to determine how far away an object is.

   Accommodation is used primarily for close objects, when there is a great deal of change in our eye muscles. Because humans don't look as much at close objects, we don't use accommodation as much as other animals. Some animals, such as a chameleon, rely almost entirely upon accommodation. When a chameleon looks for a fly to eat, it must focus right in front of it, and uses accommodation to determine the fly's distance. If a chameleon were given glasses that refocused its eyes, it would never be able to catch the fly, thinking it always too close or too far away.

Convergence

   Convergence measures depth by measuring the direction your eyes are pointing. When you focus on an object, your eyes cross at a certain angle. The closer this object is, the larger the angle. As an object gets further and further away, your eyes almost become parallel. Your brain keeps track of the convergence angle of your eyes, and can use this to determine your distance from the object on which you are focusing.

   Like accommodation, convergence is most useful at short distances, so is not used extensively by humans. However other animals rely heavily on convergence for depth perception, and there are many ways humans can easily see its effects:

	Try it Out: Look at a distant object, for example a tree across the street. Hold your finger right in front of your face and focus on it. This will cross your eyes, greatly increasing your convergence angle. While focusing on your finger, look at the tree. Because your convergence angle has increased, the tree will appear closer and smaller.

Parallax

Parallax creates depth through observed changes as you move. Close one eye so that you are only looking through one lens. Hold out your thumb in between you and another object, such as this computer screen. There is some part of the screen that is obscured by your thumb. However by moving your head, you can see around the thumb to the screen. When your brain uses parallax, it gauges the relative movement of two objects as you move, which will describe the relative distance between you and the two objects.

An interesting result of Parallax can be seen in the British artist Alfred Leete's World War I recruiting poster. When your brain sees the picture, it processes the image of Lord Kitchener and interprets it to be three-dimensional. However if you move to the left or right, there is no difference between the relative movement of the outstretched hand and the head behind it. The brain compensates for this by making the picture appear to rotate as you move so that the finger and eyes are always pointed directly towards you.

Go Back to Binocular Disparity

Move on to Depth Ambiguity