How does Zebra actually work? - General Discussion and Assistance - CHDK Forum

How does Zebra actually work?

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Offline albo

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How does Zebra actually work?
« on: 10 / June / 2008, 12:18:22 »
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Hi folks,

I'm curious about the zebra algorithim.  How does it tell if a pixel is "underexposed"?  Does it just look to see if the overall brightness of that pixel (basically the number of photons received by that ccd element) exceeds a high limit, or goes below a low limit?  I'm a bit of a photog newbie, so i think i'm not understanding some of the basics here.  If one puts a black square and a white square underneath the exact same lighting conditions, the camera might consider one as underexposed, while the other would show up as overexposed?  Basically I'm wondering - how do color and lighting interact to make something "under" or "over" exposed?  Taking a picture of a black field, would the camera bascially try to adjust f-stop, iso and shutterspeed settings to receive the same amount of light as if it was taking a picture of a white field?  If so, then over/under-exposure has nothing to do with how a human "sees" the picture?

thanks for your attention...

allie

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Offline fudgey

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Re: How does Zebra actually work?
« Reply #1 on: 14 / June / 2008, 16:31:59 »
Zebra is a tool, not a god. You decide the exposure, the zebra doesn't  ;). When/if it's working properly, it will tell you where in the scene it's darker or lighter than whatever the resulting digital image can represent using the current exposure settings. If there are zebra stripes over an area where you want details to be stored in the photo, zebra has given you the clue that you will need to alter the exposure.

If you are shooting something that can't be managed by autoexposure and auto white balance (like your all black or white walls with no detail), you really shouldn't use those automatic modes. You should either use manual modes or the paint bucket in photoshop instead.  :haha

Also, the zebra in chdk doesn't count photons, it just reads what's on the camera's LCD and trusts that (while half-shuttering) the camera is guessing how the exposure will turn out well enough.

 

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