Why does looking at things from a small hole make it more clear?

  1. Hello everyone,

    Why do looking at things with one eye closed through a small hole like microscope make the object look clearer. Someone told me it gives less abberation. Then I checked it online and it said different wavelengths converge at different points in a lens. Ok I understand why that would create a blurred image, but I don't understand why looking at objects as the way I describe create less abberation. Thanks :smile:
     
  2. jcsd
  3. The hole behaves like a new source, this has to do with diffraction
     
  4. And effectively everything before that is a miss match of destructive and constructive interference patterns.
     
  5. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Not sure about the microscope, but if you mean a small hole just in say a piece of card, or a hole made with your finger

    That's correct.
    The equations you learned for lenses and focus only apply to rays parallel to and near the axis, as you get further off axis the lens (including the one in your eye) can't focus them all to the same point.
    There are 4-5 main aberations but they all get worse as you go to larger apertures, and some of them as the size of the aperture ^2 or ^3
     
  6. Thanks a lot for your reply :smile: I don't know much about lenses. When I checked the definition of abberation, I thought it meant different colours of light don't converge at the same point in a lens. So how does creating a pin hole make different colours of light converge at the same point. How is this creating less abberation? Thanks a lot for your time :smile:

    Edit: Oh wait do you mean, that when there is a pinhole, the light rays would be coming parallel, so their is more convergence at one point?
     
  7. jtbell

    Staff: Mentor

    When you look at an object "normally," rays from the object enter your eye and its lens through the entire area of the eye's pupil, and the lens focuses the rays onto the retina. Because of aberrations in the lens, the focus isn't perfect.

    The pinhole reduces the area of the lens that is actually used, which reduces the amount of aberration. This is a general principle. It also applies to cameras: when you reduce the aperture (for a given lens), you generally improve the sharpness of the picture, until the aperture becomes so small that diffraction becomes noticeable.

    Notice also that you can usually see better outdoors in bright sunlight than indoors or in the shade, because the pupil of your eye shrinks in bright light.
     
  8. mgb_phys

    mgb_phys 8,952
    Science Advisor
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    That's just chromatic aberation, there is also
    Spherical aberration
    Coma
    Astigmatism
    Field curvature

    A small aperture helps with chromatic aberation because with a small pupil the light all enters the lens at a small angle to the curved surface (on axis - the lens surface is flat to the light) so the light 'sees' a smaller prism effect and is dispersed less.
     
  9. Thanks Jtbell :smile: it certainly helped. Ok so is the answer for this question,

    Edit: Also what creates aberration in the first place. Is it the diffraction of light as it travels in the air that makes hit the axis at an angle?
    1.when there is a pinhole, the light rays would be coming parallel, so their is more convergence at one point?
    2. Less light rays less abberation

    I think it is both but both of you have emphasized the second one. Is it simply that more light rays you have higher the chance of abberation. Is it simply that not much physics in this. Thanks :smile:
     
  10. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    For chromatic aberration it's that light hitting the surface of the lens at an angle sees it as a prism and gets diverged.
    For most of the other aberrations it's that, for a spherical lens, light hitting the lens parallel to the axis doesn't com to a focus in the same place as it gets further from the axis.
    Even for a parabola, light entering the lens at different angles doesn't all com to a focus in the same place.
    It's simply a matter of geometry, by limiting the amount of off-axis ligth that can go through the lens you are blocking the rays that will form the worst images, and so improving the image you get.

    Then in the real world there are also the effects of lenses not being perfectly shaped, or being mounted slightly tilted or off center.
     
  11. Oh I get it. Thanks mgb_phys :smile: It was all about distance from the axis. Thanks again :smile:
     
  12. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Unfortunately the wiki page isn't very clear.
    But most optics books will have pictures of the different aberrations.

    You can generally reduce most of them given enough lens element (each extra bit of glass gives you a couple of extra parameters to adjust) and if you are allowed to use fancy (expensive!) aspherical or high-index lenses. Thats why the lens for a Hassleblad or a Leica costs more than the one on your cell phone.
    But the small aperture of the lens on your cell phone is how a 1cent bit of moulded plastic is able to give any sort of image at all!
     
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