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Thread: White balance and Exposure

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    Alis's Avatar
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    White balance and Exposure

    The White balance changes when the exposure setting is changed. I thought it would be interesting to discuss it's meaning. I feel I know why but I am not able to get my head around it's exact meaning

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    Re: White balance and Exposure

    I don't think it actually changes - I think it has more to do with noise from the sensor (i.e. you'll be a bit off if you w/b in a drastically underexposed scene). If you whiteballance your image at the correct exposure - you can tweak your exposure widely and the w/b should still be correct.

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    Re: White balance and Exposure

    Quote Originally Posted by Alis View Post
    The White balance changes when the exposure setting is changed. I thought it would be interesting to discuss it's meaning. I feel I know why but I am not able to get my head around it's exact meaning
    It shouldn't, unless you've blown one of the channels, although on gray sampler patches on a colour card you often get a slightly different temperature on the brightest one (only by the numbers - nothing that you'll see)

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    Re: White balance and Exposure

    Quote Originally Posted by Alis View Post
    The White balance changes when the exposure setting is changed. I thought it would be interesting to discuss it's meaning. I feel I know why but I am not able to get my head around it's exact meaning
    Were you using auto white ballance or doing a custom white ballance? I'm curious why it would be changing for you.

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    Re: White balance and Exposure

    White balance modifies global exposure (to a minimum), but global exposure doesn't modify white balance.

    Regards

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    Re: White balance and Exposure

    Thanks, guys.

    So, Colin you are suggesting that it is because the software does not have usefull information on the blown channel, it can not decide and sets the wrong WB?

    Or as _GUI_ (sorry don't know your first name), it is only a one way, so if I set the exposure and then adjust the white balance, I have to go back and readjust the exposure, albeit by a tiny amount.

    I know this is a tiny amount but I was curious to know what is the relationship.

    Kent, yes, that is one way to know it happens.

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    Re: White balance and Exposure

    Ahh ok. I take it you arn't using a white card then. The camera's software is probebly weighting the scene differently as you adjust your exposure. The actual kelvin temp of the light isn't changing though, so it's more of a camera malfunction (or at least imperfect software).

    If you are using a white card there are two ways to get a slightly wrong reading, the first as I origionally suggested, a drastically underexposed scene, and the second as Colin suggested, one or more of the channels are being clipped.

    White ballance can affect exposure -- "sort of". When you adjust the wb slider in ACR, you are sliding the RGB channels against eachother. You *can* slide one of the channels passed the clipping marks on either side (This usually wouldn't be a ballanced image, unless it is dominated by a single color). Thus you must adjust the other sliders (brightness, contrast, recovery, fill, etc.) to bring the detail back into acceptable values. This is all post-production though.

    The colored sensors on the camera are pre-whiteballance, so the actual exposure is not affected by it at all. The white ballance is simply a value used to instruct the software how to combine the three gray channels (RGB) into a color image. You can think of it as the opposite of using the channel mixer for color to b/w conversions. All white ballance in-camera is a preview. Setting a custom white ballance in the camera can save a lot of headaches in post if you don't have anything neutral to w/b against. When the camera is set to auto, it is rare that all of your shots in the same session will have the same white ballance and shift - again, another reason to use a white card.

    Does this help at all?

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    Re: White balance and Exposure

    Quote Originally Posted by KentDub View Post
    Ahh ok. I take it you arn't using a white card then. The camera's software is probebly weighting the scene differently as you adjust your exposure. The actual kelvin temp of the light isn't changing though, so it's more of a camera malfunction (or at least imperfect software).

    If you are using a white card there are two ways to get a slightly wrong reading, the first as I origionally suggested, a drastically underexposed scene, and the second as Colin suggested, one or more of the channels are being clipped.

    White ballance can affect exposure -- "sort of". When you adjust the wb slider in ACR, you are sliding the RGB channels against eachother. You *can* slide one of the channels passed the clipping marks on either side (This usually wouldn't be a ballanced image, unless it is dominated by a single color). Thus you must adjust the other sliders (brightness, contrast, recovery, fill, etc.) to bring the detail back into acceptable values. This is all post-production though.

    The colored sensors on the camera are pre-whiteballance, so the actual exposure is not affected by it at all. The white ballance is simply a value used to instruct the software how to combine the three gray channels (RGB) into a color image. You can think of it as the opposite of using the channel mixer for color to b/w conversions. All white ballance in-camera is a preview. Setting a custom white ballance in the camera can save a lot of headaches in post if you don't have anything neutral to w/b against. When the camera is set to auto, it is rare that all of your shots in the same session will have the same white ballance and shift - again, another reason to use a white card.

    Does this help at all?

    Thanks, Kent. May be you explained this, but I am still not sure how changing the WB can change the exposure.

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    Re: White balance and Exposure

    It really dosn't

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    Re: White balance and Exposure

    Quote Originally Posted by Alis View Post
    Thanks, guys.

    So, Colin you are suggesting that it is because the software does not have usefull information on the blown channel, it can not decide and sets the wrong WB?
    It's a theory of mine - but in practice;

    (a) I try not to over-expose my highlights, and

    (b) I try to remember to use a gray card for deathly accurate white-balancing (if required) anyway.

    In "real world" terms I'd suggest "just don't worry about it" - shoot RAW - nail your exposures - and you'll normally get pretty close with Auto WB and guaranteed accuracy when using a grey card (which you should be using anyway )

    There isn't really a relationship between the two that I'd personally invest any time on.

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