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"Dynamic iris" and contrast ratios

Discussion in 'Projectors, Screens & Video Processors' started by alanfcross, Feb 12, 2005.

  1. alanfcross

    alanfcross
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    Just trying to get my head round the manufacturers' creativity when it comes to specifying contrast.
    I see that some are using an automatic variable iris to maximise contrast (at least in the spec sheet!). Am I right in saying that this will be a full-on / full-off ratio when fed with peak white and black in turn? With an iris doing half the work it seems to me that you might achieve these high contract figures in theory, but never in the same image frame. Or is the real situation better than this?
     
  2. Gary Lightfoot

    Gary Lightfoot
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    I think you've summed it up very nicely.

    When quoting contrast ratios when there's an iris involved, it's often referred to as 'Dynamic CR' to distinguish it between ordinary on/off CR without an iris.

    The iris will be fully open for bright white, and fuly closed for deep black, so that's how the dynamic figure is derived. If they kept the iris in one position (turned off) for both readings, they'll probably measure something like 1400:1 at best.

    Gary.
     
  3. moco

    moco
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    In practice, though, the contrast for one frame is not so important.
    You want to have a good picure for dark scenes and you also want a good picture for bright scenes.
    A picture that contains both a very bright part and a very dark part is rare.
    You don't get good contrast in those cases with your own eyes either, because the iris in your eyes also adjusts per the intake of light.
     
  4. Gary Lightfoot

    Gary Lightfoot
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    That's a very good point.

    When I use Avia to set the white and black levels, I use the needle pulse tests. What I find there is that using the dual screen that has both white and black levels makes it difficult to adjust the black level because my eyes iris is closed, and I can't see the black bars easily. Because of that, I set the white level there, then move on to the black screen with the two moving bars and move back to check the white. That way I can more accurately set the black level.

    Of course, it's debateable that doing it that way may mean you won't see the shadow detail in scenes that have bright components to them (due to your iris). If you then up the brightness to make black level detail more visible, you degrade your black level (it's now grey), and your overall contrast ratio has come down. You will also exascerbate any mpg artefacts and dark area dither.

    I personaly always set the black level using the black screen and take my chances. :) I think the gains doing it that way outweigh the losses of the other.

    Gary.
     
  5. moco

    moco
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    small correction: you should have said "because my iris is closed". Our eye's iris opens to enlarge our pupil when it is darker, so that more light enters. It is the opposite of a projector's iris. It is the same as a camera's iris.
     
  6. Gary Lightfoot

    Gary Lightfoot
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    Yup, my bad. Edited accordingly. :)

    Gary.
     
  7. David_of_Surrey

    David_of_Surrey
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    A thought....
    What would happen if you lit a small candle at put it on a coffee table between you and the screen? would the change in your eye's iris caused by the flame make you see a darker image on the projector screen?
    Dave
     
  8. cyberheater

    cyberheater
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    Isn't that the theory of bias lighting behind TV's/Projector Screens etc...

    To force your eyes not to open up full wack on dark scenes to give the impressions of better blacks.
     

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