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Thread: Question about WB tutorial

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    ldasignup's Avatar
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    Question about WB tutorial

    I found the WB tutorial to be very helpful:
    http://www.cambridgeincolour.com/tut...te-balance.htm

    However, I'm not sure I completely agree with one of the statements made:
    > "Of course, do not try to change your composition to include a colorless object, but just be aware that its absence may cause problems with the auto white balance."

    Well maybe with film in the "bad old days", but with extra digital captures being essentially free, I'd tend to zoom to include an object that appears to be a neutral gray, and take its picture. Then use that to set the WB in PSE/PS/ACR/LR/etc with an eyedropper and sync with the "intended" image.

    For example, with the tutorial scene with white/gray boat in the foreground, if I was using a DSLR crop camera with 18-55 kit lens, I might use 18mm for the overall scene, and then 55mm for the boat. Or I might take the trouble to walk closer to the boat, if possible, so it took up more of the picture.

    My 2 worth.

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    Re: Question about WB tutorial

    Quote Originally Posted by ldasignup View Post
    I found the WB tutorial to be very helpful:
    http://www.cambridgeincolour.com/tut...te-balance.htm

    However, I'm not sure I completely agree with one of the statements made:
    > "Of course, do not try to change your composition to include a colorless object, but just be aware that its absence may cause problems with the auto white balance."

    Well maybe with film in the "bad old days", but with extra digital captures being essentially free, I'd tend to zoom to include an object that appears to be a neutral gray, and take its picture. Then use that to set the WB in PSE/PS/ACR/LR/etc with an eyedropper and sync with the "intended" image.

    For example, with the tutorial scene with white/gray boat in the foreground, if I was using a DSLR crop camera with 18-55 kit lens, I might use 18mm for the overall scene, and then 55mm for the boat. Or I might take the trouble to walk closer to the boat, if possible, so it took up more of the picture.

    My 2 worth.
    Hi Lynn,

    Welcome to CiC - great to have you with us.

    I think you're reading something into the tutorial that wasn't intended ... what you're suggesting is fine; I think what the tutorial is saying (put another way) is "don't 'not take the shot you want' just because it doesn't have a spectrally neutral reference". Or yet another way; "Don't feel like you have to recompose so that your shot includes a spectrally neutral reference ... just be aware that if it doesn't though, then auto-WB may be a bit further off than usual".

    Hope that helps

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    ldasignup's Avatar
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    Re: Question about WB tutorial

    Agree.

    However, I'd also be fussy about another phrase:
    "then auto-WB may be a bit further off than usual"

    I don't use auto-WB that much, but my understanding of how it works is that it averages the entire scene. The presence or absence of a smallish white/gray object won't necessarily influence the auto-WB that much to make much difference.

    Actually, I often wear a checked/plaid shirt that is reasonably close to neutral gray when taking pictures where I think I'll be concerned about WB.

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    rpcrowe's Avatar
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    Re: Question about WB tutorial

    I shoot AWB all the time and often, but not always, include a WhiBal card, shot under similar lighting conditions to give me a reference for my white balance...

    I am fortunate in that many of my images include my Maltese rescue dogs. The white coats of these dogs make great white balance targets. Since the dogs are the primary subject of the images, I want their coats to be white!

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    Re: Question about WB tutorial

    Quote Originally Posted by ldasignup View Post
    my understanding of how it works is that it averages the entire scene.
    Not sure where you got that from.

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    ldasignup's Avatar
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    Re: Question about WB tutorial

    Quote Originally Posted by Colin Southern View Post
    Not sure where you got that from.
    This may be obsolete info for older cameras, but I have the impression that it works out the calculated Temp+Tint WB that causes the entire image to average out to a neutral gray. Perhaps it is more intelligent than that, and only uses part of the image. At any rate, the intent is to identify, "hmmmm ... there seems to be a color cast in the image .... let's neutralize it".

    I did come across this explanation:
    > "The simplest automatic white balance mechanisms look for the brightest point in the scene and assume it is white and adjust accoridingly."

    What is your understanding of how AWB works?

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    Re: Question about WB tutorial

    The gray-world thing is explained in a paper I've put up on my site here. (pdf, about 4MB)

    It starts with:

    the so called Grey-World algorithm which as well as being one of the oldest and
    simplest is still widely used. This algorithm has been proposed in a variety of forms by a number of different authors [5, 18, 21] and is based on the assumption that the spatial average of surface reflectances in a scene is achromatic. Since the light reflected from an achromatic surface is changed equally at all wavelengths it follows that the spatial average of the light leaving the scene will be the color of the incident illumination.
    The paper goes on to discuss several AWB algorithms . . enjoy.
    Last edited by xpatUSA; 30th November 2012 at 04:53 PM. Reason: typo+emphasis

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    Re: Question about WB tutorial

    From past readings the WB is based on temperature averages across the entire frame. That sensor generated data is applied against the software in the camera and the data is adjusted accordingly. The matrix table of changes is simply the best estimates for auto of someone in the manufacturers Engineering department of the average temperature expected under certain lighting conditions.

    The light needs to be measured accurately by the sensor to provide data to allow the software to make changes using that matrix. The higher quality the sensor the more accurate the luminance is determined. Software protocol, taking the Bayer array into account, will calculate the temperature and make the adjustments as programmed by the software the Engineer configured or the firmware you modified. You can generalize Tungsten 3000K, Fluorescent 4000K, Daylight 5000K, Cloudy 6000K, Shade 7000K. Obviously there are +/- variations applied as can be shown with multiple fluorescent settings but the major steps highlight the process. The algorithms make the adjustments required to present the jpeg photo in auto closer to the 5000K to 5500K range human eyesight expects as the norm. Too orange, add blue. Too green, add magenta. When shooting in raw you override some of that software and you control the eventual WB output to a jpeg rendition.

    Other on-board software searching for the pure white or pure black to use in conjunction with a Auto WB is to be expected with protocols for that too. Similarly we see face recognition software (pre-determined temperature spots in the frame) that comes into play not only with WB but focus as well.

    That's my own understanding as best I can determine from my reading.
    Last edited by Andrew1; 30th November 2012 at 05:31 AM.

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