What is it?
Somewhere in the course of Evolution certain animal species developed eyes that could converge. If on the one hand these animals, that include us humans, lost the incredible coverage angle of 360 degrees provided by opposite eyes, on the other hand they became capable of seing in depth, or stereoscopically (in greek, "solid vision").
In order to understand stereoscopic vision and its importance to survival, one just has to close one eye and try to accomplish simple everyday tasks in this way. The smallest gesture, such as reaching an object on the table becomes a challenging operation under monocular vision. The main obstacle will be perceiving depth and estimating the distances separating objects from the observer. Even though monocular vision still relies on other depth cues, such as perspective laws.
The peculiar element in stereoscopic vision in called parallax, the combining of images from different viewpoints. The three-dimensional way we see the world results from the interpretation, by the brain, of two bidimensional images it receives from the eyes.
Human eyes are separated in average 64mm from each other and are capable of convergence and divergence, crossing their axis as close as few inches ahead the nose or as far as infinity, when they become parallel. Other species’ opposite eyes obviously never converge.
In addition to images from the retinas, the brain also receives information from the muscles that move the eyes about their degree of convergence or divergence. This is the most important information the brain needs in order to accurately evaluate distances on the depth axis.
Stereoscopy was first described by Sir Charles Wheatstone in 1838, not far from the invention of photography. It didn’t take long untill stereoscopic photography was invented. Obviously, it quickly became very popular because of the fact that the addition of depth improves the sensual aspect of the visual experience, in other words, the pleasure of seing.
The principle of stereoscopic photography is simple and immutable: two photographs of the same subject are taken from viewpoints slightely apart. It gets complicated when it comes to the viewing, when each eye has to see only the image that is meant to it and not the other, and for this pourpose countless different techniques were developed.

How to view the images in this page
In order to view stereoscopically the images reproduced in this page, you’ll need a mirror (a little bit bigger than each of the images, and not much bigger than your monitor screen). Place the mirror vertically, in a right angle to you monitor, one of its edges between each stereo pair, the reflective side to the left. Place your head close to the monitor so your nose touches the other edge of the mirror and look at the right image. You will see both images superimposed.

Can viewing stereo images ruin my eyes?
The observation of stereoscopic photographs can’t do any harm to your eyes. On the contrary, it’s a good exercise to the muscles that move the eyeballs and can stimulate the stereoscopic perception of the world. One estimates that from 10 to 15 % of the population who can see with both eyes have lost stereoscopic perception in some degree (1).
On the other hand, since these images are reproduced in an electronic medium, monitor radiation becomes a relevant source of eye discomfort, especially during a close and prolonged observation, which doesn’t have anything to do with stereoscopy itself. If you have a good printer, we advise you to view them on paper.


(1) Lipton, Lenny, Foundations of the Stereoscopic Cinema – Van Nostrand Reinhold Co. – 1982, pp. 77-79.