You Do Have Double Vision, You Just Don’t Notice It…


If you have two eyes, you have double vision. Each eye sees a different view of the objects in front of your eyes.

Try this experiment.

Two blurry pencils are one pencil seen with double vision

Get two pencils. Hold one pencil about 18″ away from your face slightly to the right and hold the other pencil at arms length slightly to the left. While staring at the closest pencil and keeping it in focus, make an effort to become aware of the pencil in the distance. Hey, there are two of them back there. What’s up with that?

Now look focus on the pencil in the distance and stare at it. No, there aren’t two pencils back there anymore. But wait! While holding focus on the pencil farthest away you now can become aware of two pencils being held about 18″ away (if you didn’t move).

Now pencil in front is doubled

If you are like most people, you never noticed that you had double vision because your brain fixates on the object you are focusing on. Everything else is perceived as a type of background noise that we simply don’t pay any attention to.

Notice also that the pencil that you aren’t focused on isn’t doubled exactly. One eye will be slightly dominant over the other eye and you will perceive one of the doubled pencils with what appears to be a level of transparency that differs from the other one.

Guess what? The farther away you put the pencils the less you notice the double vision. But it is still there out to about thirty – fifty feet at which point the difference is so subtle that you brain can completely mask the disparity and you can’t perceive it as distinct double vision.

For a multi perspective photograph, something radically different happens. Your eyes move apart and closer together as they look at things in the 3D photo that appear closer and farther away, but they stay focused on the surface of the photograph. The suppression of double vision is a different experience because the focus doesn’t change. As the difference increases (close to far away) it actually can become uncomfortable for some people to look at since the brain is expecting to focus the eyes differently for something up close and compared to something farther away. But just like the perception conflict of the inner ear to vision for astronauts floating in zero G, with a bit of practice – and keeping the amount of distance depicted not that great at first – it can be accommodated by most people.

The same is true for motion pictures. If the producers don’t get crazy with the amount of depth they try to create and have stuff appear to come way out of the screen, this taxing of the brain to overcome perception conflict doesn’t have to reach headache levels.

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17 Comments

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17 responses to “You Do Have Double Vision, You Just Don’t Notice It…

  1. Kavan

    Dear Mr. Almont,
    My name is Kavan and I study at the univ of Florida. Your notes on double vision are very interesting.
    I may have some simple questions that I would like to ask you.

    Q1: So we have double vision naturally, because of our two eyes. But when we focus on a particular object we don’t see two images of it.
    We don’t get double vision from the object we are focusing on because the differences between retinal images are solved by the eyes changing their convergence (to overcome the double vision).
    In what sense? What does it really mean that our eyes change their convergence? Do they move the two images together in the brain? Does the brain fuse the two images?
    It seems to be a choice (due to limited computational ability) that the brain does to work on the two retinal images from that specific object and not on the retinal images from the other objects we are not focusing on….

    Why do we instead get double images from the objects we are not focusing on? Why doesn’t the brain not fix those double images too?

    you mention that “…One eye will be slightly dominant over the other eye and you will perceive one of the doubled objects as slightly more transparent than the other…. ”

    Why is one of the double images more transparent? In what sense and why does the more “”dominant” eye get a fuller image?
    Q2: What is the horizon? Why do parallel lines appear to converge to a (vanishing) point that lies on the horizon (which is a horizontal line)?

    • Fusion happens when the brain is able to process two images as a single image with depth. Typically, this requires a single point in space that both eyes are focused on and converged upon. Items in front of that converged point and items beyond that converged point at some distance or disparity point in space are no longer within the range of being perceived fused into a single image with depth. At that point, those items become unique and double vision is a result. The brain, it would seem, has some point of correlation whereby if there is too much disparity or difference between what the left eye sees and what the right eye sees then fusion does not occur and double vision is the result. But fusion isn’t the only thing going on. Having two points of view as provided by two eyes makes it possible to “see through” objects in front of you, like leaves in a forest for example. What obscures one eye can be seen by the other eye, and the brain makes the obscuring object transparent. Try it for yourself and hold a small object in front of you and see how it becomes transparent when you look at objects farther in the distance.

      Dominance for one eye over another is similar to dominance of one arm over another. To be left or right handed, etc. There has been much written about this. I would encourage you to do a search and read a few articles.

      Parallel lines converge because of the angle of the eye to the line. If you align one eye to a line in front of it, it stays straight. It is the angle of the eyes in relation to the lines that makes them appear to converge which is different than simply the distance between lines appearing to be smaller as distance increases. It is easy to understand that items in the distance look smaller. This is different than taking into account the angle each eye presents to what it is converging and focusing on.

  2. Kavan

    Thanks Mr. Almont!
    great answers: I get both the transparency issue and the double fusion one now…
    a few more last comments: when we watch TV, both our eyes receive two different, flat, 2D, images. Why don’t we see TV as 3 dimensional? Why do the eye not produce the depth effect from the two different images?
    Is stereoscopic the same thing as binocular vision?
    What is exactly panoramic vision? How is it different from peripheral vision?
    thanks!
    Kavan

    • When we look at TV each eye sees the same image in terms of parallax and focus. We focus only on the surface of the TV and each eye sees the same perspective so no depth infomation can be processed by the brain… EXCEPT… for secondary depth cues from motion parallax and inferred depth from occlusion and size and shadow. However, most TV screens are so small that even these cues are not very powerful. “Stereoscopic” implies seeing with stereopsis and fusing two images into a single image with depth. Binocular vision is seeing with two eyes without regard to stereopsis. Panoramic vision is where the eyes are located more on the sides of the head.

  3. Kavan

    Thanks Mr. Almont!!
    So when we look at a car on the street our two eyes see two different images of the same object. These images are different in the following ways:
    each eye can sees more or less the same amount of the whole object. But due to parallax, cues and other background objects around the focused object are different. These different cues show up in the images formed by each eye. The two different retinal images get fused in a single 3D image.

    When watching TV instead, the scene on the screen is somehow slightly different for each eye (since they are a little displaced from each other). But each retinal image is practically the same image since the background is the same for both retinal images…

    Here I show my ignorance about parallax:
    it is an apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight. The apparent displacement of an object caused by a change in the position from which it is viewed. We have two eyes and to each eye the object is located in a different position based on the fact that its background is different for each eye…Also, as the viewer’s viewpoint moves side to side, the objects in the distance appear to move more slowly than the objects close to the camera. Is this what parallax means?

    Kavan

  4. Kavan

    I have also read about panoramic vision:
    Prey animals are said to have a more panoramic type of vision of the world: they get almost 360 degree field of view . Animals have their eyes on the side of their heads….
    Does it means that those animals don’t really see in 3D like us?

    Humans and other predators have a more stereoscopic vision. The field of view is much smaller (~ 60 degrees)….

    Kavan

  5. That Means That The Blurry Double Thing I See Is Double Vision! Ive Only Had 1 Eye Test Which Was The One With The Weird Black Out Glasses And The ABC, I Got 1/1 So My Mum Doesnt Take Me To Optitions Because She Thinks Ive Got Good Eyes. I See Light In My Vision Too. I See Double Vision Alot But My Friends Dont. When Ever I Have It Everything Goes Blurry. I Tried On My Friends Glasses And It Actually Helped A Bit. Do I Need Them?

  6. It Hurts My Eyes When I’m Looking At A Car Go By Or When Im In THe Car Watching The Trees Go By

  7. rhiyaa

    why do i experience a slight double vision when something is moving very fast

    • Double vision implies that you are perceiving two different images that aren’t fused into a single image with depth. This actually happens all of the time, for example, when you converge your eyes on something up close, things in the distance have too much disparity to become fused into a single image with depth and you have double vision for those things in that field of view. As to things moving fast, there could be any number of things going on and you should seek a vision care professional for evaluation or mention it at your next eye doctor’s appointment to determine if what you are perceiving is abnormal. Mark Changizi has written about vision evolving based upon suppression of of one eye over another when things in the foreground obfuscate things in the distance. He calls it our x-ray vision and it provides a greater field of view in an environment like a forest. His books are an interesting read. You need to determine if what you are seeing is a normal part of seeing or something that is problematical due to fusion problems. A good resource is COVD.org

  8. Mike Davis

    Is the phenomena you describe above what occurs in the movie Patch Adams, (YouTube video here: http://www.youtube.com/watch?v=CBDgLL2de_c)? Thanks for your response.

  9. Steve

    Hi when I try this one of the ‘ghost’ images is taller than the other – is this a normal phenomenon when taking this test or should they be perfectly aligned?

  10. Laura Stratton

    Hi I have had double vision since I was eight and I’m now in my thirties. I have tried exercises and injections into the eye and prisms but nothing has stopped the double vision. I have recently read about physiological diplopia when researching.
    I am wondering if I have this and if there is a way I can stop myself from being aware of double images in the front and behind what I am focusing on. For example if I am sitting in a room full of people and focusing on a speaker at the front the peoples heads in front of me are double.
    I find it annoying but also distressing and am really hoping I can do something to suppress the images.

    Many thanks

    Laura

    • I’m not a doctor, but if I were you I would seek out an optometrist who specialized in vision therapy using tools and techniques to help with fixation on a single point. There are many useful techniques and eye exercises. These can be incredibly effective because the human brain has amazing capacity for adaptation through exercise and therapy. Sue Barry was able to see 3D for the first time in her life when she was 40 years old. Ophthalmologists told her that was impossible. They were wrong and continue to be wrong even today. Good luck! According to Sue it was totally worth it.

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