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White Balance
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Color Temperature
Most light sources are not 100% pure white but
have a certain "color temperature", expressed in Kelvin. For
instance, the midday sunlight will be much closer to white than the
more yellow early morning or late afternoon sunlight. This diagram
gives rough averages of some typical light sources.
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Type of Light |
Color Temperature in K |
| Candle Flame |
1,500
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| Incandescent |
3,000
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| Sunrise, Sunset |
3,500
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| Midday Sun, Flash |
5,500
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| Bright Sun, Clear Sky |
6,000
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| Cloudy Sky, Shade |
7,000
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| Blue Sky |
9,000
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| White Balance
Normally our eyes compensate for lighting
conditions with different color temperatures. A digital camera needs
to find a reference point which represents white. It will then
calculate all the other colors based on this white point. For
instance, if a halogen light illuminates a white wall, the wall will
have a yellow cast, while in fact it should be white. So if the
camera knows the wall is supposed to be white, it will then
compensate all the other colors in the scene accordingly.
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Most digital cameras feature
automatic white balance whereby the camera looks at the overall color of
the image and calculates the best-fit white balance. However these
systems are often fooled especially if the scene is dominated by one
color, say green, or if there is no natural white present in the scene
as show in this example. |
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The auto white balance was unable to
find a white reference, resulting in dull and artificial
colors.
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The auto white balance got it right
this time in a very similar scene because it could use the
clouds as its white reference.
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Most digital cameras also allow you to choose
a white balance manually, typically sunlight, cloudy, fluorescent,
incandescent etc. Prosumer and SLR digital cameras allow you to
define your own white balance reference. Before making the actual
shot, you can focus at an area in the scene which should be white or
neutral gray, or at a white or gray target card. The camera will
then use this reference when making the actual shot.
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