Peeve: Stereopsis and depth perception
16 October 2005, 12:02 PM
I see this regularly in stories, but I also hear people say it regularly in real life, and I just noticed that I hadn't posted about it before.
It's widely believed that someone or something with only one eye has no depth perception.
However, this widespread belief is simply wrong.
If you don't believe me, try this simple experiment: Close (or cover up) one eye. Hold your hand up in front of your face. Can you tell that your hand is closer to you than other objects in the room? Now hold up both hands, one further away from the other. Can you tell which one is closer to you? Now move your head from side to side. Do closer objects appear to move faster than objects further away?
The answer to all three questions is almost certainly yes, because depth perception is only partly about stereopsis. Binocular vision gives you better depth perception than monocular vision; but even if you have only one eye, you can still perceive depth, because there are clues other than binocular parallax. For example, things that are further away appear to be smaller. And things that are closer can occlude things that are further away. And if you move your head, or the objects in question are moving, monocular parallax provides clues as well. Oh, yes, and lighting cues help too. Among other things.
For much more about this, see the section on monocular cues in that Wikipedia article on depth perception.
Comments
Yes, yes, and yes. I know someone who has vision in only one eye who is one of the better drivers I know; this could hardly be the case if he had no depth perception.
Posted by: Nao![[TypeKey Profile Page]](http://www.kith.org/journals/jed/nav-commenters.gif)
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October 16, 2005 5:49 PM
Depth perception is suprisingly persistent! I have this congenital eye thing called Duane's syndrome which means that my eyes don't work as a team, I'm crosseyed without glasses, and I have no peripheral vision on my right side. But! I have excellent depth perception.
Posted by: SarahP (mail) | October 16, 2005 6:15 PM
True, but not as funny.
Posted by: David Moles (mail) (web) | October 17, 2005 9:49 AM
When I talk about having lousy depth perception (and I do), I don't usually mean an inability to tell which of two objects is closer to me, I mean an inability to estimate how much closer to me the closer object actually is. When driving, that often means that if I'm making a left turn, I have to wait for oncoming cars to get fairly close before I can have a sense of whether the gap between them is big enough to get through, by which time it's usually too late to try anyway. I also can almost never tell, when watching a fly ball, whether it's an easy out or a home run, but I think most people can't tell that (unless they do what I do and watch the outfielder rather than the ball). I also am lousy at telling how fast something is moving toward me, which also makes me a lousy driver as well as a lousy outfielder. Of course, none of those things are due to monocular vision, in my case. Mostly, I expect it's that I'm lousy (or just slow) at interpreting the cues you discuss.
I will add, though, that I understand that it's easier to trick a person with one eye (or a camera) with forced perspective. That is, when the cues you are talking about are deliberately misleading, two eyes will be more likely to tell you something is wrong, and that the spaceship is not, in fact, a long way away but is just really small. However, this may really be because forced perspective requires a stationary viewer at a specific point, and it's easier to get a camera to stand still and stop squirming around.
Thanks,
-V.
Posted by: Vardibidian (mail) (web) | October 17, 2005 11:41 AM