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White balance correction means to adjust the illumination in a photo to white. In order to accomplish this one must have a neutral in the image like a gray card or a white wall. Software can adjust white balance, but not all software are equally good.
The first thing you might like to take care of, when you set out to edit your photos, is white balance. White balance refers to the color of the light and assumes that the optimal light color is white. Some pictures, like sunset or candlelight, do not have white light, but usually an impression of white light is the best. Some use Photoshop\’s auto levels to set white balance, but that is not a good idea, since auto levels only sets the brightest pixels to white and the darkest areas to black without looking at the mid tones. But what if the brightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most images have black areas, but the lightest pixels are rarely pure white).
When correcting white balance, the mid tones are the most important and in order to set the mid tones correctly one adds a grey card to the image when taking the photo. A grey card is a sheet of cardboard or plastic toned an exact mid tone neutral gray. Ideally one has three cards: a black, a gray and a white. Photoshop\’s levels adjustment panel has three color pickers for picking color: one for white, one for gray and one for black. By clicking the gray color picker on the gray card, one can set the mid tones to neutral gray. One can only include a gray card in the picture if one intends later to crop the photo.
If one does not want a gray card in the picture, or if one doesn\’t have a gray card at hand, one can later use specialized software that scans the image and finds the color of the light and sets it to white. There are draw backs with such applications: what if there are no neutral areas in the picture to calculate the color of the light from? Some applications do not need a neutral in the photo, but most do to get a good result.
When working with RAW images, you will have found that RAW converters usually come with a slider for color temperature, which means a slider to adjust the picture cool or warm. But what if the color of the light is greenish as when you have taken a picture in neon light? The cool-warm slider is good for regular incandescent light, but not for fluorescent.
Color adjustment controls are rarely good for correcting white balance, because the color correction will not only neutralize the gray card, but will also tone the picture in an undesirable way: often the blacks get toned or the whites or both. In short one needs some neutrals in an image to adjust white balance. A white wall or a sheet of white paper will do well; preferably add a gray card for the mid tones.