Stereo vision Qs

One of my eyes (my left) is very much stronger than my the other. If I cover my left eye then reading a newspaper, say, is very difficult. I remember my optician telling me when I was younger that the resulting lack of binocular vision and depth perception meant I would “never be a test pilot or a Test cricketer, but you can see perfectly well”. In fact all eye tests I’ve had show that my vision with both eyes is above average.

I’ve often wondered to what extent I’m missing out. To me, the world doesn’t look appreciably “flatter” through one eye than it does through two. TV and films don’t look especially “flat” either - I’ve never seen the point of 3D glasses, which don’t work for me anyway.

So, a few questions to those of you with “normal” eyesight to help me figure it out:

  1. Does your depth perception diminish when you close or cover one eye? I assume my brain uses other cues to figure out distances (size, speed etc), so I can still catch a ball, play squash, drive etc.

  2. Hold a pen (or a finger) up close in front of your eyes and focus past it so that you can see two images of it. Are they both equally distinct? I get a clear one on the right (from my left eye) and a ghostly, faint one on the left (from my weaker right eye).

  3. Still with the pen in front of your face, focus on it. Close your right eye and move the pen so it appears to be in front of, say the edge of the monitor. Then close your left eye - the pen will appear to move to the left relative to the background. Now open both eyes - is the pen in the “right eye” position or the “left eye” position, or is it somewhere in between? When I do this, with both eyes it appears to be in the same position as with my left eye only. I take this to mean that my brain is getting rid of the weaker signal from my right eye. Is this correct?

Hopefully some of you are bored enough to play around winking at the screen and satisfy my curiosity… thanks :slight_smile:

IANA oculist.

  1. I have heard that the main purpose of two eyes simply is to have one in reserve in case you lose one. No kidding. Two eyes just aren’t required for 3-dimensional vision, as your experiences show; as long as you’re not standing still totally, even one working eye can give you enough pictures for the brain to combine them to a 3D-perception of your environment.
  2. With me, it’s just as you say.
  3. I’m getting two faint ghostly pictures of my finger, one in the left eye position, one in the right one. They’re both not very distinct though.
  1. I totally lose my depth perception when only one eye is open. Yes, there are other cues, as the OP wrote. One of the cues not listed there is sharpness – faraway things are a bit fuzzier, without such sharp edges. Painters use this technique a lot.

  2. Equally sharp.

  3. This experiment, as described in the OP, is a good demonstration of what (I think) is meant by depth perception. When I focus on the monitor, I see two fingers, each in the same position as with one eye shut. On the other hand, the other eye picks up the text which is behind the finger, and the result is two areas of the screen which are dimmer because of the finger which is superimposed on it.

But this is an artificial situation, which doesn’t occur much in real life. More commonly you’ll be focusing on the finger, and you won’t consciously realize that the text on the screen is unreadable, because there are two copies of it superimposed on each other but skewed by an inch or so. Of course, as soon as you move your eyes back to the monitor, there’s only one version of it, easily readable – except for those two darned phantom-fingers in the way.

If the finger is closer to the monitor, then part of the text will be blocked from the vision of both eyes. But each eye will catch a millimeter or so of text which the other eye is not able to see. What’s even less obvious is that each eye will see a tiny bit from the side of the finger which is hidden from the other eye, because of the angle that each eye sees it from.

In other words, the right eye can see some of the right edge of the finger, and the right border of the totally-blocked area of the screen. And the left eye can see some of the left edge of the finger, and the left border of the totally-blocked area of the screen. This I refer to this as the ability to see “around” the finger. It is what signals the brain that the finger is closer than the screen, and I think that this is what “depth perception” is all about.

Besides the experiments listed in the OP, here’s another one: Have you ever tried looking at the pictures (especially cartoons) in a ViewMaster Stereoscopic Viewer? If find the 3D effect in those to be rather exaggerated, and might help you to understand what 3D vision is about. Being a static, unchanging picture, you might be able to experiment in ways that this per-and-monitor bit don’t allow. Good luck!

A spare eye? I doubt it (sorry).
My bet is that while we can function with only one eye, we hunt better with two, so we have evolved two.

My eyes don’t work together either. An opthalmologist checked it out once and said my eyes are not tracking together or focusing on the same vertical point.

The result is my depth perception is awful. Don’t let me parallel-park your car unless you like dents. Visual acuity, though, is excellent at 20/20, and on a good day, approaches 20/15.

The world doesn’t look any different to me out of one eye or both - with two eyes, it’s wider, but that’s it.

TV and regular movies look flat because they are flat 2-D images.

3-D movies give me double vision and headaches. The last time I got dragged into one with a group at Disney, I had the choice of wearing the glasses and keeping one eye closed or not wearing the glasses and seeing a mess. Needless to say, I had no idea exactly what was happening when the rest of the audience eeked or shrieked. Likewise the Viewmaster. I never understood what all the hoopla was about.

Answering your third question - you’re left-eye dominant, meaning you primarily use that eye, and you’re using the right eye more or less for peripheral vison to the right.

The world doesn’t look any different to me, either (one eye vs. two) until I attempt to do something that requires depth perception.

Try tossing wadded-up sheets of paper into a trash can 15-20 feet away. Do it with both eyes open to set a baseline for yourself.

Now close one eye and try it.

My own accuracy (distance-wise) goes completely to hell. I can tell as I’m trying to throw that my brain doesn’t “know” how far away the garbage can is, and is guessing. It’s a surprisingly awkward feeling.

I’ve known some people who claim not to notice much difference, but this has convinced me that I definitely use the depth perception from binocular vision when I’m doing something like that.