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Optics: how to make eye focus in distance?

  1. Jul 23, 2014 #1

    I have a theoretical & practical question.


    I noticed that when I turn on my smart phone camera application and look 'through it' on some distant object, holding smart phone 30 cm from my eyes, then my eyes are focused on 30 cm and not in distance.

    Is there a way (i.e. using a computer program) to make smart phone camera application be like a transparent glass, i.e. so there will be no difference looking to a distant object through glass or through smart phone via camera?

    And now the practical question:

    For people with myopia (especially children) reading books worsens the myopia. When reading, a person holds book about 25 cm from eyes and his/her lens muscles are tensed which raises pressure inside the eye and eventually (with myopia people only) cause the eye-ball to become longer and increases myopia.

    The solution for this problem would be to project an electronic book to a distant screen and read from this screen. But this is not so convenient, because in this case you cannot read where ever you want, you are bound to the room with the screen and the screen should be huge etc.

    Why instead not to use a computer program which displays on a normal iPad the content of the book, but causes the eye to focus in distance, similar to what the Google glasses do?

    I am wondering if there is such software and how it should work from geometrical optics point of view.
  2. jcsd
  3. Jul 23, 2014 #2
    You can't achieve this with software, but a lens between the screen and the viewer (at a distance approximately equal to the focal length of the lens from the screen) will do it.

    Products have been available for some time to achieve this - search for "full page magnifier" - and they work just as well with a book!
  4. Jul 23, 2014 #3


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    Staff: Mentor

    Hi leonid.ge! If you display a stereogram image on the screen then it can be arranged that your eyes must focus at a long distance in order for you to gain the 3D effect needed to perceive the desired image. The text of a book's page could be displayed as a stereogram image, thereby forcing the reader to totally relax their focus in order to read it. (There are ways to stop them cheating and closing one eye then focussing the other on the screen to read as they normally would!)

    A google search on "stereogram" should turn up a wealth of material. It's a fun topic. You use software to construct the image.

    Good luck!
  5. Jul 23, 2014 #4
    For the 'full page magnifire' that you mentioned - this is just a magnifying glass that is useful for people fith hyperopia (who cannot focus their eyes enough to see close objects).

    In fact I am looking for an opposite effect: focus your eyes in large distance, but see things on computer screen which is close to you. I don't really understand why this cannot be achieved with software. It seems to me that you can run software which will model images of the letters on computer screen as so they are in distance.

    The observer will have to look 'through' the screen, as though he/she looks at something in far distance behind the screen. It is similar to watching 3D autostereoscopic images (Google 'Autostereoscopic text' images).
  6. Jul 23, 2014 #5


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    The techniques you are referring too, do not change the focus of the eye lens (which is relevant for myopia). They create the depth effect by changing the focus of the looking direction of the two eyes, while the lens focus is still on the near screen.
  7. Jul 23, 2014 #6


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    Staff: Mentor

    That's a good point. It could explain why people (children, in particular) have such difficulty in learning how to view stereograms---focussing near while not turning the eyes inwards is something we've never needed to attempt before in real life.
  8. Jul 23, 2014 #7


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    In normal viewing there is a direct relationship between the two different focus settings (lens, stereo), that the brain learns alowing it to adjust the lens and stereo simultaneously. Decoupling the two different focus settings is unnatural and could be responsible for eye-fatigue and headaches, that many experience from 3D effects.
  9. Jul 23, 2014 #8
    Exactly. They can only focus on images which are further away (e.g. at infinity), which is exactly what you are looking for. Note that the magnifying effect is only a by-product of the lens's principal action which is to create a virtual image at a greater distance - if all it did was magnify then the letters would be bigger but just as blurry.

    Because when your eyes are focused at infinity light rays that are parallel are focused on your retina. Light rays from an object closer to you are diverging and therefore do not form a focused image on your retina: it does not matter what pattern is displayed on the screen, if the eye is focused at infinity it will see a blur.
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