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Question About Light and Nearsightedness

  1. Mar 25, 2009 #1
    I'm not sure if this is the proper type of physics to post this under, but here I go:

    If you're nearsighted, take your glasses off, with four fingers form a tiny square-like peeking hole, and bring the hole up to your eye. You'll notice the images in the distance become sharper as if you put your glasses back on.

    The reverse, by my account, isn't true. If you form the square and look through it with your glasses on, the distant objects are as clear as before. I'd think it should be fuzzy (the same way if someone who isn't nearsighted wears your glasses, he sees nothing but blobs), but that seems not to be the case.

    How does this happen?
     
  2. jcsd
  3. Mar 25, 2009 #2

    A.T.

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    Put you glasses back on and read the title of this forum :smile:

    Lenses and holes create pictures in different ways. Lenses bend light coming at different directions, and can bend it too much or not enough creating a blurry picture. Holes just reduce the light to one direction, blocking most of it, therefore creating a dark picture.
     
  4. Mar 25, 2009 #3

    jtbell

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    Moved from the relativity forum to a more appropriate one.
     
  5. Mar 25, 2009 #4

    Lok

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    It is not the proper sort of physics... anyway the answer is that peeking through a small hole will always sharpen your sight. Just like a pinhole camera, the smaller the pinhole the sharper the image but it will lower the luminosity of the camera. U can say that the smallest patch of different color ( like a pixel) will be of roughly the same size as the pinhole ( the distance from the observed object is also important).

    There are some special glasses that consist of black plastic with tiny holes in it, they can sharpen anyones vision. They act like sunglasses without the UV protection.
     
  6. Mar 25, 2009 #5

    Andy Resnick

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    As Lok points out, you have made your eye into the equivalent of a pinhole camera. Pinhole cameras have a numerical aperture near zero (but still much larger than a wavelength), so the depth of field is very large.
     
  7. Mar 25, 2009 #6

    DaveC426913

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    This is a secondary function of the f-stop aperature setting on a camera. Close the aperature down and you get a sharp image (and also a longer focal range).


    BTW, I have an "emergency" pair of glasses next to my bed that I made just for fun. It is nothing more than a business card with a cutout to fit over the bridge of my nose and a pinhole for each eye to see through. It works well enough that I can read news articles on TV without the need for my glasses. :biggrin:
     
  8. Mar 26, 2009 #7
    so far i've been told that it's the equivilant of a pinhole camera and that small holes sharpen images, but that doesn't have anything to do with physics. The same finger question i asked applies to the camera question. How do small holes focus light?
     
  9. Mar 26, 2009 #8

    DaveC426913

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    In a nutshell, it is not that small holes focus an image, it is that small holes block parts of an image that would be out-of-focus.


    A focused image is the result of light rays converging on the focal plane; an unfocused image results when not all the light rays converge there. Some rays converge in front of- or behind- the focal plane.


    Now, light rays from an object you are looking at pass through all points of your cornea and lens before converging on your retina. Because your eye is not perfect, some rays fall in front of your retina, some fall behind. The upshot of this is that the image is blurry. This is as your eye is normally.

    However, when you block all those rays except a very few (say, a pinhole or with your fingers), then light rays coming from the object pass through only a very small area on your cornea and lens. Less passage through the imperfect medium means less errant rays of light. Your lens can adjust to ensure these few rays do converge on your retina. The net effect is that there are no light rays any longer that converge in front of- or behind- your retina. Thus, sharp image.

    See attached diagram.


    And BTW, yes, cutting out all that light means that an image through a pinhole is much dimmer than through a larger aperature. This is why focus and light-gathering needs are at-odds in all optics (cameras, eyes, telescopes, etc). Every exposure is a compromise of these two elements.
     

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    Last edited: Mar 26, 2009
  10. Mar 26, 2009 #9

    turbo

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    I learned that trick almost 50 years ago, from the town's barber. He was far-sighted with very poor close-focus capabilities and he told me that he used this method to be able to see well enough to read a phone book when his car broke down and he had left his glasses home. He knew that I was interested in science when I was pretty young, and had to share that trick with me. I told lots of people about it, and some of them were pretty doubtful until they tried it themselves. Years later, I bought a metal body-cap for my OM-1 with a laser-drilled hole in a brass insert in the center. Got some interesting exposures with a locking cable release on the B setting. The images were soft, with great DOF.
     
  11. Mar 26, 2009 #10

    DaveC426913

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    When I was a teen and dreamed of being the next John Dykstra, I photgraphed an 18" scratch-built spaceship model to make it look full-sized by getting extremely close and cranking up the depth-of-field so its entire length was in-focus (a large depth-of-field is what makes small things look large).

    By poking a pinhole through a sheet of foil I got an aperature of approxiately f300. This allowed my depth-of-field to range from zero inches to 18". The model was physically touching the camera lens and yet still in focus. I was pleased. I've still got the pic around somewhere. Model too.
     
  12. Mar 26, 2009 #11

    turbo

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    If you're still interested in pinhole photography, here's a good one to try. Find a stream with nice cascade-type falls and trees. Shoot on a windy day so the trees and leaves will be moving. The water and the trees will be soft and motion-blurred, and the rocks in the stream will be quite well-defined. I've got a shot like that around here somewhere, but in all the 10,000+ prints, it would be impossible to find, or I'd scan it and post it. Pin-hole orifices are pretty much one-trick ponies, but depending on how you use them, it can be a nice trick.
     
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