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Question about camera lens taking a picture

  1. Jul 2, 2015 #1
    Hey guys,

    Can someone give a basic explanation of why a camera can take a picture even if half of its lens is covered up?

    Thanks guys...
     
  2. jcsd
  3. Jul 2, 2015 #2

    Drakkith

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    In the image below, the red lines represent light rays from an object directly in front of the lens and located far away so that the light is very nearly parallel when it enters the lens. If you block off the outer rays, perhaps by closing the iris diaphragm, you still have rays entering the camera so you still have an image. Perhaps the key thing to understand is that all of these rays originated from a single point on the object. So if you took a picture of your friend, where the center of the image is at his chest, then these rays all came from a single point on his chest located directly in front of the center of the lens. The rays that came from his nose would be tilted at an angle when entering the lens (imagine tilting all the red rays so that they are traveling downwards slightly). The lens would then focus all of those rays onto a spot at the focal plane further down from the center.

    Does that make sense?

    optics-kids-lenses-fig1.gif
     
  4. Jul 2, 2015 #3
    Okay, thank you I think I get it. Also, do the light rays bend due to refraction?
     
  5. Jul 2, 2015 #4

    Drakkith

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    Yes they do.
     
  6. Jul 2, 2015 #5

    CWatters

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    Aside: "Covering" the lens is effectively what the iris in the eye or aperture in a camera does. It's primary effect is to alter the brightness of the image.
     
  7. Jul 2, 2015 #6

    Andy Resnick

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    It's worth pointing out that this thread assumes that the lens is covered symmetrically- stopping down or conversely, introducing a central obscuration (catadiopteric lenses). Obscuring the lens in a non-axisymmetric fashion leads to various optical processing techniques, such as:

    Foucault knife-edge test
    Schlieren imaging
    Aperture synthesis
     
  8. Jul 3, 2015 #7

    davenn

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    agreed, Andy

    when I first read the OP stating the lens was 1/2 covered I was considering a blockage coming up say from the bottom so only the top 1/2 was passing light

    optics-kids-lenses-fig2.GIF

    Drakikith assumed a symmetrical aperture closing arrangement ... not sure if that was what the OP had in mind ?
     
  9. Jul 4, 2015 #8

    Drakkith

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    That's a possibility. Still, the basic explanation doesn't change. Rays of light are still getting through even with half the lens covered so there's still an image formed.
     
  10. Jul 4, 2015 #9

    davenn

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    OK :) I was trying to visualise that
     
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