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How can squinting be used by both a myopic & hyperopic eye?

  1. May 26, 2012 #1
    I am a myope, and I can squint to focus what normally would be a distance that is beyond my range of focusing. I figure that this is due to the eyeball being deformed, making the profiles of the cornea & lens be less pronounced - or perhaps by shortening the eyeball (which would seem to be the cause behind making the profile of the cornea & lens less pronounced.) But I am wondering how a hyperope also squints to focus at a distance less than the range of focusing, since the action of squinting would seem to make their eye more myopic.

    Perhaps squinting is something that can be done in both directions, and as a myope, I have learned to only do it my way, whereas hyperopes learn to do it in the opposite way?
     
  2. jcsd
  3. May 26, 2012 #2

    ZapperZ

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    This sounds like the pin-hole effect.

    Zz.
     
  4. May 26, 2012 #3

    Cleonis

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    I concur with ZapperZ's remark.

    The squinting creates circumstances that are alike to a pinhole camera.

    Here is another way of trying it out.
    Tak a piece of paper, preferably rather stiff paper, and punch a small hole, something like 1 or 2 milimeter. Take a look through that hole, holding the paper as close as possible to your eyeball. You'll notice the myopia is somewhat alleviated.

    This test shows that the effect does not involve deformation of cornea or lens. As in the case of looking though a small hole, squinting reduces the aperture.
     
  5. May 26, 2012 #4
    OK, I think I understand. A pinhole camera, by definition, allows for an any image distance as there is a one-to-one correspondence of points on the object and on the image at any image distance. This works by simply not allowing light from anywhere but that corresponding point to reach the image surface.

    When coupled with a regular lens imaging system, the pinhole removes all the light except that which is coming from a specific point (and thus in a specific direction), and thus causes the image (i.e., at any image distance) to be precise. Of course, since a perfect pinhole camera is impossible (i.e., since it would have an infinitely small area), a very small aperture camera still helps as it restricts the area from which emitted light makes its way to the image surface, thus reducing the blur from any bad focusing - which in essence, makes the perceived focusing better.
     
  6. May 27, 2012 #5

    Cleonis

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    There what is called 'depth of field'. A pinhole camera has in effect total depth of field. Objects in the foreground cast a sharp image on the screen, and so do objects in the background. This is because a sharp image is cast for every distance of the screen to the pinhole.

    In the case of a camera lens we have that at the center of the lens the front and rear surface are close to parallel to each other. That is, at the center of the lens the shape of the lens is close to that of a flat sheet of glass. Light rays that traverse the center of the lens are not broken. (More precisely, when light traverses a flat sheet of glass the refractions at the front and the rear pretty much cancel each other.)

    The shape of the camera lens is such that it allows a large aperture, and still a sharp image is cast on the screen. That way more light is gathered.
    The cost is loss of depth of field. The larger the aperture, the shorter the depth of field. Of course actual camera's have complicated lens systems, but the property that a smaller aperture gives a larger depth of field still holds.


    It just occurred to me that it may be possible to devise a contact lens that alleviates myopia; a pinhole contact lens. A pinhole contact lens would leave you with poor light gathering capacity, but I think that in the daytime there is plenty of light anyway. (Downside: night vision would be poor to non-existent.)
     
    Last edited: May 27, 2012
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