Question about aperture.

Discussion in 'Beyond the Basics' started by dbrandon, Jan 2, 2008.

  1. dbrandon

    dbrandon TPF Noob!

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    Kind of a 'physicys' question i guess, but here goes ... what is it that causes the 'star' effect on lights at smaller apertures compared to larger apertures where the effect is far less (for example in night time shots) ?

    The effect i mean can be seen here ...

    [​IMG]

    (i know it's not a good pic, but it has the effect i mean in :wink:)


    So yeah, does anyone have a quick explanation to why the small aperture amplifies this effect, or should i take it to my physics teacher ? :)
     
  2. jdgreen92086

    jdgreen92086 TPF Noob!

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    You may not like it, but I really do. As to your question, I don't know. I just wanted to tell you I like your picture :)
     
  3. hamster

    hamster TPF Noob!

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    I think it's the leaves of the aperture closing in and getting tighter that cause the star effect.
     
  4. azruial

    azruial TPF Noob!

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    "The number of blades in an iris diaphragm has a direct relation with the appearance of the blurred out-of-focus areas in an image, also called Bokeh. The more blades a diaphragm has, the rounder and less polygon-shaped the opening will be. This results in softer and more gradually blurred out-of-focus areas. In a photograph, the number of blades that the iris diaphragm has can be guessed by counting the number of spikes converging from a light source or bright reflection. For an odd number of blades, there are twice as many spikes as there are blades.
    In case of an even number of blades, the two spikes per blade will overlap each other, so the number of spikes visible will be the number of blades in the diaphragm used. This is most apparent in pictures taken in the dark with small bright spots, for example night cityscapes." -wikipedia

    Found this on Wikipedia, although the article is a bit lacking in references. It's not a detailed description of the optical physics involved, but still relevant and (IMO) interesting. I second what hamster said, I think it is somehow caused by the places where the aperture leaves intersect and those angles are different at different apertures...

    I'd love to hear a more detailed optical explanation of the cause if you get one from your teacher or find a link!
     
  5. dbrandon

    dbrandon TPF Noob!

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    Thank you jdgreen92086 ! :) To be fair, i don't dislike the picture 'that' much, i just wish the car pulling out didn't have to stop right before the exposure had finished ! :grumpy:

    Hamster and azruial, thank you for your replys. Clearly the wikipedia extract (which definately is relevant, thanks) suggests that it is related to the diaphragm blades. Also, i too would have to say it must be, as hamster said, the differences in the angles / shape / something like that !

    I'm gonna have a Google around and see what i can find :thumbup:
     
  6. dbrandon

    dbrandon TPF Noob!

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    From what i've read the lines are caused by defraction of light from the aperture blades. At smaller apertures more blade is exposed, meaning a greater potential for defraction.

    The effect is more evident on night scenes due to the contrast between the intensity of light, and the (often) dark background. That seems obvious, but i had overlooked the fact that maybe the starring happened in day time pictures too :)
     
  7. Helen B

    Helen B TPF Noob!

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    Yes, as the iris is stopped down the spread caused by diffraction at the aperture blades increases and the ratio of the diffracted light to the undeviated light rises. What should be a point of light at the image plane would turn into a disc surrounded by rings of decreasing intensity if the iris was a perfect circle. Because the iris is a series of intersecting nearly-straight lines, the disc and rings are turned into the star shape.

    Normally only the central disc (called the Airy disc) is important, but when you have very bright objects like light sources the brightness of the limbs of the star becomes significant.

    That was a highly simplified explanation, but I hope that it helps in some way.

    I would suggest that you still discuss it with your physics teacher, no matter how convincing or unconvincing an explanation you find on the web.

    Best,
    Helen
     
  8. dbrandon

    dbrandon TPF Noob!

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    That definately does help Helen, thank you. You certainly know what you're talking about ! And i dread to think what i'd make of an un-simplified explanation :)

    As you suggest, i will probably still speak to my physics teacher about it.

    Thanks again :wink:
     
  9. azruial

    azruial TPF Noob!

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    It was also an elegant and well-worded explanation, thank you :D
     

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