I've read that the brain can calculate 3D by taking two images from two eyes and comparing them. However, 3D works perfectly fine with one eye. Surely this "Binocular vision" is pointless if the brain can calculate 3D perfectly fine with one eye.
Does the brain just guess or is there some other trick involved? I notice eyes individually focus. So the brain could calculate the amount of blur and work out the distance that way?
Best answer:
Answer by carrotstien
Hello. Good question.
Our main method of determining optical depth is parallax. Each eye sees the world from a slightly different angle (as you know). If you close one eye, your ability to perceive depth is severely diminished. Try to play catch with a soft object with one eye closed - you should find it quite difficult.
Of course, as you have realized, it is not impossible to perceive "3d" using one eye. However, this is due to shortcuts we learn throughout our lives. We know rules like shadows, overlapping, angular motion during motion, focus, memory of size, etc.
However, these are all very negligible with respect to depth perception compared to parallax.
This is exactly how optical illusions like these two work:
dragon head:
http://video.google.com/videoplay?docid=7338400765555658225
taller twin illusion:
http://spriteland.files.wordpress.com/2009/06/mens-height-illusion.jpg
Using focus sensitivity to determine depth isn't a valid method - while being better than the stuff mentioned above, it still much worse than parallax. The reason is that parallax is more or less linear. Something twice as far away, will split off twice as far apart when looking at something nearer. However, the blur when using one eye, doesn't change linearly - in fact, just looking at something blurry, won't give you a hint at how far it is other than letting you know which very general area it is in. Something twice as far, while looking at a close object, will not seem to be twice as blurry - that is the problem.
Again, this doesn't mean that people with one eye will live horrible lives. It just means that they will not be baseball players :)
My grandmother, who lost vision in one eye, is fine, and has become accustomed to the situation. However, while it already stopped bothering her, I still see her sometimes grab for thin air when reaching for something.
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