We live in a 3D world, and we perceive a 3D world. However, how we actually achieve 3D vision is an intriguing psychological question. After all, we only have a2D retina, so where does the extra dimension come from?
The most common answer to this question is to do with the fact that we have two eyes. Having two eyes means we get two slightly different views of the world. This can be seen by simply closing one eye after the other and switching views between them. You can see that objects in the world appear in different positions relative to each other. This is called "binocular disparity" and is the basis of depth perception. If you notice, objects that are nearer to you seem to shift position in your visual more than those in the background do. The brain takes the information from both eyes and combines it to interpret these disparities and give us a sense of depth. This method of perceiving 3-dimensions is known as "stereopsis" or "stereoscopic vision". Charles Wheatstone demonstrated stereoscopic vision experimentally in the 1800s, with his invention of the stereoscope, now often used at fun fairs and similar places. Wheatstone took two photographs of the same scene using cameras whose lenses were a few centimeters apart. This produced two pictures corresponding to the images formed on the retinas of each eye. Wheatstone's stereoscope consists of a peephole for each eye through which the pictures can be seen – the "left picture" through the left peephole, and the "right picture" through the right peephole. An observer will see a 3D image when looking through the stereoscope. This demonstrates that disparity produces a sense of depth.
However, people with only one functioning eye are still able to perceive a 3D world. Such people are obviously not using stereopsis. Also, not everyone develops stereoscopic vision. Children with a squint - or strabismus – (a slight misalignment of the eyes) do not develop stereopsis unless their condition is corrected very early in life, yet they can perceive depth too. There must be other ways to perceive in 3D than just stereopsis.
Hermann von Helmholtz proposed that we use a series of "cues and clues" to perceive the 3D world. The cues and clues he identified were:
No one of these methods of seeing the 3D world is used alone. They all have their strengths and weaknesses. The most effective, however, is stereopsis. After all, we evolved to have two eyes for a reason. None of the methods are really able to directly represent the 3D world. All the information we use is detected on the 2D retina, therefore any sense of three dimensions we have is constructed by the brain. The brain can reconstruct the 3D world incorrectly at times (for instance with the pseudo-3D advertisements painted on sports fields, and with Wheatstone's stereoscope). In reality, we do not really perceive the 3D world at all - we infer it.
