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