Working with Contrast and White Balance in Digital Photography
by John Hudson
When editing images you ought to first address white balance and contrast. White balance is usually what you would look at 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 addresses the color of the illumination within the photograph and sets white as a goal. White balance applications attempt to correct the color of the illumination to white and to do that, the application needs some neutrals in the picture to calibrate the suitable correction tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are more difficult to find in real life, so one can use a dedicated gray card.
White balance software usually has both a manual and an automatic mode. 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 converting RAW pictures, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Automatic white balance correction usually needs neutrals in the image, like a gray-card or white-card or both. Some applications can dispense with that, 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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The usual way to manipulate contrast is simply by changing the difference between the individual red, green and blue values and the middle value (128); like this: R= (R-128) * contrast + 128; and likewise for the green and blue channel. If the image is very dark or very pale this method will not do. What about very dark or very pale images? In that case you change the algorithm to: R=(R-RAverage)*contrast+RAverage where RAverage is the average red channel value of the image. And so on 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: 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.
This article is based on this about <a href="http://ezinearticles.com/?White-Balance-in-Photo-Retouching---Pros-and-Cons-of-Software-Solutions&id=7612799">white balance</a> and this about <a href="http://www.articlesbase.com/digital-photography-articles/editing-image-contrast-6544139.html">contrast</a>
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New Unique Article!
Title: Working with Contrast and White Balance in Digital Photography
Author: John Hudson
Email: jan@janesmann.com
Keywords: contrast, white balance,digital photography, photography,hobbies, arts and entertainment
Word Count: 529
Category: Hobbies
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