Why aren't photos overexposed in the middle?

Discussion in 'Beyond the Basics' started by Meysha, May 27, 2005.

  1. Meysha

    Meysha still being picky Vicky

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    Ok this question has been bugging me for a few months and I can't figure it out.

    Why aren't photos over exposed in the middle of the picture?

    From what I understand the shutter opens from the middle and goes toware the edge (Like our pupils when we go from a bright room to a dark room) so therefore the middle of the aperture sees light first.

    As it opens further the light enters into these 'newly opened' areas of the aperture. (But, remember light is also still hitting where it first hit - the centre) So I would expect the edges of a photo to be darker than the middle if this is true, because well, the middle gets more light.

    Can someone please help me understand why this just doesn't happen?
     
  2. Chase

    Chase I am now benign! Staff Member Supporting Member

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    I think you're confusing the aperture with the shutter motion. I believe most of the modern cameras use a focal plane shutter that actually moves across the frame either vertically or horizontally. Having said that, I'm probably wrong! :lol:
     
  3. Meysha

    Meysha still being picky Vicky

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    Oops,, yeah. I meant the way the shutter moves.

    Well even if it does go from one side to the other - why isn't one side brighter than the other?
     
  4. LittleMan

    LittleMan TPF Noob!

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    hmm...... good question.... I have no idea but maybe it's because it's such a short ammount of time between when it starts opening and when it's finished being open.... it's not like it's slow... so maybe that's it.

    *That's my guess*
     
  5. Chase

    Chase I am now benign! Staff Member Supporting Member

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    As it moves across, each portion of the frame receives light for the same amount of time. The part that is exposed first is also covered first as the shutter closes. Hopefully that makes sense
     
  6. Chase

    Chase I am now benign! Staff Member Supporting Member

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  7. LittleMan

    LittleMan TPF Noob!

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    I don't understand....
    If it all folds down to one side...... say the bottom.... doesn't it all have to come back from the bottom? then when it comes back up it leaves the top of the frame open longer..... :confused:
     
  8. Hertz van Rental

    Hertz van Rental TPF Noob!

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    No,Chase is right.
    Large format lenses and some medium format lenses have leaf shutters that work a bit like the aperture - but modern 35mm all have focal plane shutters (but what's the betting KSMatt knows of an obscure one that doesn't!)
    The first shutter moves across the film very quickly - the second shutter follows after. The shutter speed determines what the time lag is. The exposure time is determined by the gap between the shutter blades so at fast speeds the gap is quite narrow (this gives a practical upper limit to the fastest shutter speed).
    The net result is that the whole neg receives the same exposure so there is no density difference.
    You can get odd effects now and then with moving objects, though.
    With leaf blade shutters the centre of the neg does indeed get more exposure than the edges. But the blades move so fast that the actual difference is so small as to be undetectable (something like 1/20th of a stop).
     
  9. LittleMan

    LittleMan TPF Noob!

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  10. Meysha

    Meysha still being picky Vicky

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    Ahh in my wrong version I imagined it going like this:
    Open -> -> -> -> (Then change direction and...)
    <- <- <- <- Close

    But there's actually two bits that go in the same direction and follow each other. (Duh!) It's so obvious now.
    Thanks.
     
  11. Meysha

    Meysha still being picky Vicky

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    Oh thanks. I was actually typing my reply when you guys all posted so it's nice to see I get it.

    What sort of effects?
    I could understand them being dim (because they're moving out of the open bit of the shutter into the bit that's yet to be open)... or could you actually have them appearing a few times in the frame without blur between if they're moving fast enough ??
     
  12. Hertz van Rental

    Hertz van Rental TPF Noob!

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    If an object is moving past the camera in the same direction that the shutter is travelling in then it can get distorted.
    Imagine if something was moving at the same speed as the shutter - one little 'slice' of it would be stretched across the frame.
    In the early days of focal plane shutters it was noticed that racing cars sometimes got elongated and their wheels went oval. Try working out why.
     

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