Curious Phenomenon

Discussion in 'Beyond the Basics' started by enki9, Jan 25, 2006.

  1. enki9

    enki9 TPF Noob!

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    Back over at another forum there was this helicopter picture posted.
    http://www.airforce.ru/photogallery/dundin/kuznetsov2004/bpk/page_07.htm

    Could anyone explain, Why do two rotor blades seem bent? In reality they are absolutely straight even when in motion. It is a photographic phenomenon, but what causes it? I know something about optics and slow shutter speeds, but this phenomenon puzzles me.

    Other pictures show the moving rotor blades just fuzzy, like you'd rather expect:
    http://www.airforce.ru/photogallery/dundin/kuznetsov2004/bpk/page_12.htm

    (I didn't get the images linked directly, so there are just the links, sorry.)
    Any reasonable explanation out there?
     
  2. Azuth

    Azuth TPF Noob!

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  3. markc

    markc TPF Noob!

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    Andrew Davidhazy exploits the same effect using a modified Polaroid to get images like this:

    [​IMG]

    The lens is replaced by a vertical slit and a string is run from the film tab to a small motor. When the shutter is pressed, the film is pulled past the slit. I was standing on a turntable rotating in front of the camera.
     
  4. enki9

    enki9 TPF Noob!

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    Thank you for the replies. I should have thought something like what was explained in the link Azuth posted.

    The moving rotors in the second photograph I linked to are, however, not bent. Is this because a slower shutter speed?
     
  5. hobbes28

    hobbes28 Incredible Supporting Member

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    There's a magic number where the blades are running faster than the shutter and it looks like the first one is on that line, where the second one has a slower shutter speed so the whole blade is blurred. Keep in mind that the RPM of the blade is greater the further you go from the center, so that first one, the center part is slower than the shutter but gets closer and passes that toward the edges.
     
  6. markc

    markc TPF Noob!

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    Oh duh. I forgot to mention swing-lens cameras like the Widelux. Using slow shutter speeds with those can get some weird effects if the subject is moving. I've wanted to own one for years.
     
  7. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    It has nothing to do with slow shutter speed. In fact, the russian photo isn't slow shutter speed at all. It's 1/500 or 1/250


    The reason behind this is that it's probably shot with an old zenit. Curtain shutter with really slow curtains. 1/30 of a second to travel across the film plane.
     
  8. markc

    markc TPF Noob!

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    I didn't say that the original had anything to do with a slow shutter speed, but the Widelux images do. Find the book End Time City by Michael Ackerman and you will see lots of images taken with a slow shutter and a Widelux. The lens swings to cover a panorama. If the shutter is slow and the subject moves, they get either compressed or stretched. You can end up with a very similar effect. This happens with people just walking by, hence the slow, not fast, shutter.
     
  9. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    My apologies.
     
  10. markc

    markc TPF Noob!

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    No problem. :D
     
  11. Torus34

    Torus34 No longer a newbie, moving up!

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    Effect due to focal plane shutter. Think of the shutter as a horizontal slit which moves from top to bottom. Also in the first shot remember that the blades on the helicopter are counter-rotating. When all blades are blurred, chances are shutter was a between-the-lens type which opens and closes radially.
     
  12. Unimaxium

    Unimaxium TPF Noob!

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    Small correction -- the RPM is the same at any point on the blade, but it is the speed (e.g. MPH) that is dependent on the radius. The outside of the blade turns just as many times a minute as the inside, but your point is still valid that the outside turns at a faster speed. Sorry, I'm just in a nit-picky mood today ;)

    My only guess as to why the second photo isn't curvy is the same as what the last poster suggested -- it was probably taken with a different camera that uses a different kind of shutter (one that works like an iris rather than a slit).
     

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