A720 IS and its overexposure-behaviour - General Discussion and Assistance - CHDK Forum
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A720 IS and its overexposure-behaviour

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Offline viewpix

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A720 IS and its overexposure-behaviour
« on: 02 / July / 2008, 10:18:26 »
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Offline RaduP

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Re: A720 IS and its overexposure-behaviour
« Reply #1 on: 02 / July / 2008, 12:00:23 »
That's not a camera mistake either, it just happens that most P&S cameras have a limited sensor color resolution (10 bits per channel), so there is not enough color space to render both the highlights and the shadows at once. What you can do is either use raw mode (which gives you a little bit of freedom to adjust the contrast and color curves, etc), or do a HDR image (multiple exposures with a different Ev).
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Offline viewpix

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Re: A720 IS and its overexposure-behaviour
« Reply #2 on: 02 / July / 2008, 12:10:40 »
OK so far. But why this always happens with white regions and no other color?
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Offline RaduP

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Re: A720 IS and its overexposure-behaviour
« Reply #3 on: 02 / July / 2008, 12:56:41 »
What do you mean by " happens with white regions and no other color"?
For me it happens with the blue sky as well (in fact, that's what will usually become overexposed).
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Offline RaduP

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Re: A720 IS and its overexposure-behaviour
« Reply #4 on: 02 / July / 2008, 13:04:01 »
Oh, I think you meant: "Why do the overexposed areas always look white, regardless of the color", right?
If that's the question, it happens due to 'blooming', which is a CCD specific thing. When 1 sensor 'pixel' is overloaded by too much light, some electrons will leak to the nearby pixels, so they will all become 'full'. And full rgb is white.

Here is a more detailed link: Hamamatsu Learning Center: CCD Saturation and Blooming
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Offline viewpix

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Re: A720 IS and its overexposure-behaviour
« Reply #5 on: 02 / July / 2008, 15:06:28 »
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Offline BB

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Re: A720 IS and its overexposure-behaviour
« Reply #6 on: 02 / July / 2008, 15:24:55 »
The blooming of the white shirt is probably a limitation of any of these Canon P&S cameras... I have had some luck though using RAW images to recover blown highlights (clouds and sky issues for me).

If you are staying in JPEG--you might also play with the Program mode and try turning contrast down (and perhaps saturation) and see if that helps the mapping to JPEG (you can also turn sharpening down here too if you are going to Post Process and don't want to fight the sharpening artifacts during PP).

-Bill
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Offline LjL

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Re: A720 IS and its overexposure-behaviour
« Reply #7 on: 02 / July / 2008, 15:25:57 »
Uh... I don't quite see how what you described could be an A720-only problem.

Of course, it's entirely possible that the A720 might have different (and possibly worse) exposure algorithms - that's one thing.

But if a scene contains a high contrast (very bright *and* very dim parts), any camera that outputs 8-bit color will either underexpose the dim parts or overexpose the bright parts. That's why HDR combining is done.

I don't know if what you asked (setting a threshold of overexposed pixels) is possible in CHDK, but I really do suspect that, no matter what you do, you'll be left with an underexposed background if you want high dynamic range on the t-shirt.

Have you tried using spot metering? That might be faster and/or more fine-tunable than using the exposure adjustment, if you practice finding the best spot to meter your photo at.
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Offline viewpix

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Re: A720 IS and its overexposure-behaviour
« Reply #8 on: 02 / July / 2008, 15:47:29 »
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Offline DataGhost

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Re: A720 IS and its overexposure-behaviour
« Reply #9 on: 02 / July / 2008, 15:50:12 »
I personally think it's a problem with ANY camera and not specifically the A720. The fact that this primarily happens with white objects/surfaces is because white reflects 'most light', it reflects all colors whereas red, green or blue objects only reflect one color and all other objects reflect a combination of colors, though not as many as white.  It's because of this that white objects are usually the brightest thing in your image and are therefore blown out first.
This can't be fixed and is a limitation of image sensors in general... that's why techniques such as HDR exist.
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