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B A question about optics: Why does a pinhole aperture work like my eyeglasses?

  1. Nov 2, 2018 #1
    i wear glasses to correct my out of focus vision. Without glasses everything is slightly out of focus. Quite by accident I looked through a tiny pinhole about the size of a pinprick without my glasses using one eye. To my amazement everything was perfectly sharply in focus. The same with the other worse eye. When I took away the paper with the pinhole it was out of focus again. When I looked through the pinhole perfectly in focus.

    What would explain that?

    Tex
     
    Last edited by a moderator: Nov 4, 2018
  2. jcsd
  3. Nov 2, 2018 #2

    Drakkith

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    It's because the pinhole reduces the diameter of the incoming beam of light. When light enters your eye, the outermost rays in the beam are generally the ones aberrated the most. Reducing the diameter of this beam greatly reduces the amount of aberration, which 'corrects' your vision at the cost of brightness of the image.

    I'll see if I can find a picture that can show this.
     
  4. Nov 2, 2018 #3
    Do you know about the signs some companies put on the back windows of their cars or trucks? It’s an image of something printed on a plastic window size appliqué with thousands of tiny holes in the plastic so the driver can see out the back window through the rear view mirror.

    Similarly, could you make a pair of glasses that are essentially opaque but with thousands of tiny holes in the opaqueness so that you are getting thousands of pinpoints of light making everything in focus?

    tex
     
  5. Nov 2, 2018 #4

    Drakkith

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    No, the hole has to be centered on the optical axis of the eye. Having multiple holes arranged all around the eye would not correct your vision since it's still letting in light towards the outside of your pupil where the aberrations are greatest.
     
  6. Nov 2, 2018 #5

    Drakkith

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    Forgive the poorly drawn diagram, but I hope it gets my point across. In the left image you can see that the incoming rays intersect each other before they hit the retina. By the time they hit the retina they form a large spot instead of a small point. That's why your vision is blurry. All the light that should be concentrated into a single point is spread out, overlapping similar spots from light coming in from different points in your visual field.

    In the right image, two outer rays are blocked. As you can see, the spot formed by the rays has been cut almost in half, substantially improving visual acuity.

    Eye Diagram.jpg
     
  7. Nov 2, 2018 #6

    DaveC426913

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    I keep a piece of cardboard on my bedside table that has two pinholes in it the same distance apart as my eyes. It also has a notch for my nose.

    When there's something on TV I want to see and I can't find my glasses in the dark, I put this thing on, and I can read the finest lettering on the TV (although it's very dim).

    I call these my Emergency spectacles.

    BTW, you don't need to make it out of cardboard; you can do it easily enough with your hand:

    pinhole-hand.jpg
     
    Last edited: Nov 2, 2018
  8. Nov 2, 2018 #7
    Interesting. I guess the next question would be how big can you make the pinhole and it still work. It must be a compromise between brightness and clarity. I assume as the hole gets bigger the less focusing effect it has.
     
  9. Nov 2, 2018 #8

    DaveC426913

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    Exactly.

    The size you can make them will depend on how bad your vision is.

    Also, whether you have short-sightedness versus astigmatism.
    For astigmatism, only some aspects of the cornea are misshapen. If you could block the right angles, you could make the holes more like slots. You'd get relatively more light without the corresponding increase in blurriness. In theory.

    Make a slot-shaped hole in a piece of foil and tape it over a hole in a piece of cardboard. One eye at a time, look through it and rotate the slot through 180 degrees. If you have astigmatism, you will likely see that some angles are in better focus than others.

    astigmatism_img3.jpg
     
    Last edited: Nov 2, 2018
  10. Nov 3, 2018 #9

    sophiecentaur

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    I remember my son had an exercise at Uni to find an optimum size for a pinhole camera for observing sunspots. The image of the Sun was not large for a 1m tube and the resolution was no better than the pinhole diameter but the image brightness was limited by a small pinhole. It was a matter of finding the compromise between the two factors. I cheated and used a +1 Diopter lens.
    PS On a bright day, the performance of faulty eyes tends to be much better than in low light conditions because your pupil shrinks in bright light - almost to pinhole size. I discovered, independently, that I could see much better when I did Dave's trick with my hand, laying by the swimming pool in the Sun without my glasses. Manual focus cameras used to have an additional scale on the focus scale which showed the depth of focus for different aperture sizes (f number) so the effect even applies to lenses as well as pinholes.
     
  11. Nov 4, 2018 #10
    The smaller the aperture, the greater the depth of field. You can find when taking photo.
     
  12. Nov 5, 2018 #11

    bob012345

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    But one usually is focused in one narrow cone anyway so different holes would line up as your eyes shift.
     
  13. Nov 5, 2018 #12

    Drakkith

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    Sorry, I'm not sure what you mean. What holes are lining up and what are they lining up with?
     
  14. Nov 5, 2018 #13

    symbolipoint

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    I noticed the exact same thing. Main reason in the simplest and most unrefined terms, is that a tiny tiny hold acts like a lens because this tiny hold is no larger.

    ( I remember making a couple of pin-hole cameras back in the time when photography actually used film, darkrooms, and enlargers.)
     
  15. Nov 5, 2018 #14

    sophiecentaur

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    Time for an experiment with some card and a pin.
     
  16. Nov 5, 2018 #15

    Charles Link

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    I find the same phenomenon in reading outdoors in bright sunlight where the pupil contracts. Otherwise I require reading glasses, because I have good eyesight, but I am far-sighted=I can not focus up close. (Reading glasses essentially put the printed page in or near the focal plane of the reading glasses and make a faraway image). ## \\ ## Outdoors in bright sunlight, I can see text quite clearly, and I don't require reading glasses.
     
  17. Nov 5, 2018 #16

    symbolipoint

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    This I believe is similar or same as the idea, "Depth of Field", but I am unsure if this truely is related to the pinhole lens or not. I believe so.
     
  18. Nov 5, 2018 #17

    Charles Link

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    @Drakkith has a good diagram that explains all of this in post 5.
     
  19. Nov 5, 2018 #18

    CWatters

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    Have you noticed that people without their glasses on tend to squint/narrow their eyes? I believe this works in the same way as a pin hole by reducing the diameter of your pupil by partly obscuring it. It may also work in conjunction with your eyelashes.
     
  20. Nov 5, 2018 #19

    Drakkith

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    Yes, I believe that's the case too.
     
  21. Nov 5, 2018 #20

    lewando

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    Perhaps an improvement in the optical signal-to-noise ratio is in play.
     
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