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White Balance Colour temperature Most light sources are not 100% Color Temperature pure white but have a certain Type of Light in K "color temperature", expressed in Kelvin. For instance, the midday 1,500 sunlight will be much closer to Candle Flame white than the more yellow early Incandescent 3,000 morning or late afternoon sunlight. This diagram gives Sunrise, Sunset 3,500 rough averages of some typical 5,500 light sources. Normally our eyes Midday Sun, Flash compensate for lighting Bright Sun, Clear Sky 6,000 conditions with different color temperatures. A digital camera Cloudy Sky, Shade 7,000 needs to find a reference point 9,000 which represents white. It will Blue Sky 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.

Auto and preset white balance problems Certain subjects create problems for a digital camera's auto white balance, even under normal daylight conditions. One example is if the image already has an overabundance of warmth or coolness due to unique subject matter. The image below illustrates a situation where the subject is predominantly red, and so the camera mistakes this for a color cast induced by a warm light source. The camera then tries to compensate for this so that the average color of the image is closer to neutral, but in doing so it unknowingly creates a bluish color cast on the stones. Some digital cameras are more susceptible to this than others.

Automatic White Balance

Custom White Balance

(Custom white balance using an 18% gray card as a neutral reference.)

Multiple illuminants with different color temperatures can further complicate performing a white balance. Some lighting situations may not even have a truly "correct" white balance, and will depend upon where color accuracy is most important. Under mixed lighting, auto white balance usually calculates an average color temperature for the entire scene, and then uses this as the white balance. This approach is usually acceptable, however auto white balance tends to exaggerate the difference in color temperature for each light source, as compared with what we perceive with our eyes. Exaggerated differences in color temperature are often most apparent with mixed indoor and natural lighting. Critical images may even require a different white balance for each lighting region. Note how the building to the left is quite warm, whereas the sky is somewhat cool. This is because the white balance was set based on the moonlight-bringing out the warm color temperature of the artificial lighting below. White balancing based on the natural light often yields a more realistic photograph. Choose "stone" as the white balance reference and see how the sky becomes unrealistically blue.

In certain cases, the color of the light is what makes the photograph. A sunset is an example. Without the rich, warm colors of the light, a sunset just isn't a sunset. Auto white balance may attempt to make adjustments to correct for the warm color of the sunset light. This would produce an image with less saturated colors or colors that were different than what the photographer saw. Auto White Balance

Image with Custom White Balance

Custom white balance After selecting Custom from the white balance setting menu, point the camera lens at something white such as a piece of paper or white shirt. You can also buy special white balance cards or a white balance lens cap. The camera will take a reading from the white object and adjust the white balance to the lighting in the room. Accessing and using the custom white balance feature varies from one digital camera to another, so check the manual for specifics. Take a few test shots after the custom white balance is set and check them on the LCD. When shooting with the Custom White Balance is complete, don’t forget to switch the camera back to Auto White Balance mode. The custom setting is usually saved until the next time you change it. References:

Week5 - White Balance  

Why custom white balance is important

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