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Why are moving objects blurred in a camera?

  1. Aug 9, 2011 #1
    Forgive me if I'm missing something truly basic about photography, but I always wondered why camera photos appear blurred if the object's moving fast.

    Isn't the speed of light large enough where, even if the object is moving rapidly, the time it takes to reach the object is negligible compared to the object's velocity?

    Thanks
     
  2. jcsd
  3. Aug 9, 2011 #2

    Pengwuino

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    Sure, but cameras don't take pictures instantly. A CCD chip, for example, integrates an exposure over a finite time interval. A typical shutter speed is something like 1/60th of a second I believe.

    So if at the beginning of the exposure, the subject is on the right hand side of the field of view and is moving to the left, the light the subject emits will initially be dumping photons on the right hand side of the CCD chip. However, as the subject moves, photons will begin dumping themselves towards the left hand side. This creates a blurry image.

    This concept is VERY apparent in astronomy and is why telescopes must do what is called 'tracking' in order to have imagines that are worth something. The dimmer the object you want to see, the longer the exposure. So if you want to see an object that is so dim that it requires a 10 minute exposure time (very common!), you have to have the telescope track objects in the sky so that it rotates with the rotation of the Earth. If you just let the telescope sit there, over the 10 minutes the star in question will move along the field of view and instead of depositing photons in a small, concentrated area of the chip (giving a crisp, clean image), it'll trail off as the earth rotates, dumping photons in a wider area making it very blurry.
     
  4. Aug 9, 2011 #3

    rcgldr

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    The amount of blurring depends on the time of exposure versus how far an object's image moves across the surface of the film (or ccd's if it's a digital camera). Depending on shutter movement, you can also get "curved" images if different parts of the image are taken at different times. The shorter the duration in exposure time and/or the slower the object image is moving, the less blurry the image.
     
  5. Aug 9, 2011 #4

    jtbell

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    Probably shorter ("faster" in photographer-speak) than that. I don't know the typical shutter speed for simple point-and-shoot cameras, but with my digital SLR I use 1/250th of a second or shorter in daylight. I use 1/60th or longer only in outdoor twilight, or indoors without flash.
     
  6. Aug 9, 2011 #5
    Excellent answer by Pengwuino.

    That's why indoor photos tend to be more blurry with a cheap automatic camera. There is less light indoors, so the camera leaves the shutter open longer to get enough light to form an image, but this gives a longer window in time for the subject to move, causing blurring.
     
  7. Aug 12, 2011 #6
    And that's why in sports photography they have camera that resenble tank barrels due to their large lens sizes...so they can gather enough light in a short enough time that the subject won't move enough to cause any blurring.
     
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