3-Dimensional Vision and Why We Have Two Eyes

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 June 2005

“God gave us two ears and one mouth so that we can listen more and talk less”, so the saying goes. But why do we have two eyes? Is it just for keeping the symmetry of the head? If you close one eye, do you notice any change apart from the reduction in the size of the field of vision?

Ordinarily, you might not notice any difference in vision when using one eye but there is a little experiment that reveals a lot. You will need two biro pens and friend to assist you. Hold on to one of the pens and give the second one to the friend.

Now ask your friend to hold the biro pointing up. With one of your eyes closed, hold your pen pointing downwards and try to touch the tip of your friend’s pen. It’s almost impossible, isn’t it? Repeat the exercise with both eyes open – it’s very easy now.

With one eye closed, we see the world in two dimensions – heights and widths only. We are not able to judge depths, that is, distances from us. For this reason, you cannot tell which of the two pens is nearer to you in the experiment. That is why it is difficult to make them touch tip-to-tip.

Each of our two eyes sees an object from a different angle. The brain uses the angular difference to work out the distance from you. Now, the farther away the object is, the smaller the difference in the viewing angle.

After a certain limit, the angle is too small for the brain to detect and at that point, we lose the three dimensional vision. This is why, for example, the all the stars in the sky appear to be the same distance away yet some are many millions of light-years farther than others.

The limit of our three-dimensional vision varies from person to person depending on how far apart their eyes are. Generally, our brain can only distinguish the angles of view if the object is less than 1,000 times the distance between the eyes.

Since human eyes are about 7 to 8 centimetres apart, the limit of three-dimensional vision is about 70 to 80 metres. After that, everything seems to be the same distance away. However, some times we are able to judge how far some objects are even when the distance is greater than this limit.

We do that using our prior knowledge of the object. The farther away an object is the smaller it appears. Thus if we are looking at two cars from over 100 metres away, we can tell which is nearer by comparing their apparent sizes. But we cannot do that with things that we’ve never seen before.

That explains our two eyes; how about the two ears? That is story for another day…but it is related to this one.