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We've all heard that we have two eyes for depth perception, and if you only have one available, you have no depth perception. But this seems to be a false claim, one that can be trivially disproven by experimentation: close one eye and walk around for a bit with just the other eye open.

You have no trouble perceiving the "depth" (distance from you) of objects around you, because the two major cues we use to perceive distance, namely parallax (the phenomenon of objects appearing to move across your field of vision more slowly when they're further away) and perspective (the phenomenon of objects appearing smaller the further away they are), are both perfectly valid with only a single eye. So what is it that's supposed to be missing, and why isn't it noticeable when walking around with one eye closed?

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    $\begingroup$ Please don’t start a question with a statement like “we’ve all heard” or “we all know”. It really pisses me off, to put it crudely. I may not have heard or know about the subject of your interest, and I strongly resent the implication that I do, or if I don’t I must be profoundly ignorant. An acceptable way of expressing this idea that something is well known is to quote a source and write something like “It is commonly asserted that…”, “It is often stated that…”, or even “It is generally regarded that …”. But leave me (as a constituent of “we”) out of it. $\endgroup$
    – David
    Commented May 11, 2023 at 19:53
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    $\begingroup$ It is noticeable. I don't think you're looking at things hard enough. For example, put your palm outstretched behind the far edge of a table a small distance away. Hold a string and hang that behind the edge. With only one eye it's not actually clear there is a gap whereas with both eyes you can somehow see a gap as your brain processes both images. It happens when looking at things in a mirror too. Another example that the effect in this photo would never be seen in real life with both eyes open reddit.com/r/Bossfight/comments/o2b8du/… $\endgroup$
    – DKNguyen
    Commented May 11, 2023 at 21:43

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You've mentioned a couple monocular depth cues; there are quite a few more monocular cues you've missed, and there are also binocular ones. Wikipedia's list includes:

Stereopsis

Convergence

and "Shadow stereopsis".

I disagree that these are not noticeable for someone who ordinarily uses two eyes; it's harder to tell depth with a single eye, especially in situations where those other cues aren't useful, like for objects that come in different sizes. You also need a lot less fine depth perception to, say, not run into a wall, than you do to thread a needle.

Where does the notion come from? I suppose some people heard that two eyes are useful for depth perception, and misunderstood that information as meaning that two eyes are necessary. People tend to make this sort of error frequently, sort of a version of denying the antecedent in the form "two eyes aid depth perception" becoming the false statement "without two eyes, no depth perception".

Related questions:

Are two eyes necessary for 3D vision?

Telling distance with one eye

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