Contrast and White Balnace in Photo Retouching


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When retouching photographs one should first look at white balance and contrast. White balance is usually the thing one should fix to begin with, then contrast.The reason for correcting white balance first of all is that you can\’t correct color contrast if the image has a color cast.

White balance concerns the hue or tone of the light in the image and sets white as a goal. White balance applications attempt to retouch the hue of the light to white and to do that, the software normally needs some neutrals in the image to find the suitable correction tint from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are ideally a dedicated gray card.

White balance software comes in two varieties: automatic and manual. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW images, one usually has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. Some applications can dispense with that, but usually neutrals are needed.

There are three kinds of contrast: hue, saturation and brightness. Normally software only has a single slider for contrast that addresses all three aspects at once. A single slider usually results in an over saturated image and gaudy colors. The best software has two contrast sliders: one for luminance contrast and one for color contrast.

The standard way to manipulate contrast is simply by altering the difference between the individual R, G and B values and the middle value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is not suitable for very dark or very pale images. What about very dark or very pale images? In that case you need to change the algorithm to use the average values of the image\’s R, G and B channels, like this: R=(R-RAverage)*contrast + RAverage. And similarly for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.

If the darkest and brightest areas are not black and white a different situation arises. If that is the case, one should also be able to expand the brightness range to reach black and white. Levels adjustment is meant for this type of correction. One can do this with Photoshop\’s levels adjustment like this: Convert the image to Lab. Select the L channel only and use Photoshop\’s levels adjustment on that channel only. Then convert back to RGB mode.

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