Contrast and White Balance problems in Photo Retouching
by Frederick Johnson
When editing photos one should first consider white balance and contrast. White balance is normally the thing one ought to fix first, 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 illumination within the picture and sets white as a goal. White balance software attempts to normalize the tint of the light to neutral and in order to do that, the application usually needs some whites or grays in the picture to calibrate the suitable correction tint from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.
White balance software comes in two varieties: automatic and manual. The manual mode usually consist of a single temperature slider for adjusting the light cool or warm. This is OK for incandescent light, but not for fluorescent light or mixed light. When opening RAW photos, one usually has a temperature slider. Apart from the temperature slider, one can also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corrected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. There are a few programs that can dispense with the neutrals, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. A single slider usually results in an over saturated image and gaudy colors. At best the software will have a control for luminance contrast and for color contrast.
The usual way to manipulate contrast is simply by altering the difference between the individual R, G and B values and the average value (128); like this: R= (R-128) * contrast + 128; and similar for green and blue. This method is only suitable for images that cover the entire brightness range. What if the image is very pale or very dark? In that case you can't use 128, but have to use the average of the individual channels in the image, like this: R=(R-RAverage)*contrast+RAverage. And similarly for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.
Another problem with contrast adjustment is that not only may the average value not be 128, but the darkest and brightest areas may not be black and white. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one's software does not offer the option to expand brightness range, one can do it with Photoshop's levels adjustment like this: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.
More information about white balance can be read here: <a href="http://www.squidoo.com/whitebalance-and-digital-photography">White balance</a> and more about contrast can be read here: <a href="http://www.squidoo.com/contrast-and-photo-retouching">Contrast</a>
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New Unique Article!
Title: Contrast and White Balance problems in Photo Retouching
Author: Frederick Johnson
Email: jan@janesmann.com
Keywords: white balance, contrast,Photo retouching, image retouching, photo editing, image editing,Photoshop
Word Count: 533
Category: Hobbies
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