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In the old days, back in the darkrooms with photo paper, there was a black and white system called multigrade. Multigrade paper came in six grades and each grade resulted in photos with more or less contrast. Today software provides contrast adjustment, but not all contrast adjustments are equal or equally good. What is contrast? Contrast is defined as (noun) "The state of being strikingly different from something else, typically something in juxtaposition or close association." It can also be defined as: (verb) "To set in opposition in order to show or emphasize differences." This means we in pictures have three different types of contrast: contrast of luminance, contrast of color and contrast of saturation. Most applications offer a single contrast slider that edits all three at once. The usual way to adjust contrast is simply by changing 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. But what if the image is generally dark or pale, then this algorithm will fail. In that case you have to use the average values of the image's R, G and B channels, like this: R=(R-RAverage)*contrast + RAverage. And so on for G and B. The problem with this algorithm, in both cases, is that images tend to become overly saturated and colorful when raising contrast. Similarly reducing contrast makes the image look like you have placed a semi-transparent gray film over it. Photoshop has an option in their contrast editing panel called: "Use Legacy". This method has the drawback that the image becomes pulled towards the primaries R, G and B if you enhance contrast and pushed towards a uniform mid-gray if you lower contrast. It is not very useful. When considering luminance contrast you can use the above algorithms after converting the RGB values to L values (luminance). Luminance is not just the average of RGB, since the channels are not equally bright. Green is brighter than blue. Therefore one usually uses a weighted conversion like this: L = R*0.299 + G*0.587 + B*0.144. This algorithm is used in the YIQ colorspace. However, there are several ways to enhance luminance contrast. First you can simply expand the range: if Lmax is 200 and Lmin 50, then you can for example expand the range so Lmax is 255 and Lmin is 0. But what if Lmax and Lmin already are 255 and 0? Then you will have to pull the existing near mid tone L values towards the extremes. This leaves two situations: one will normally lose the mid tones that way and split the photo; in stead one can use dedicated software that can preserve mid tones while pulling L values towards the extremes. There exists dedicated applications for luminance contrast adjustment. If you want to do it in Photoshop, you will have to convert the photo to Lab mode, make the L channel alone active and adjust contrast for that. Remember not to use Legacy mode or the image will tend towards either black and white or uniform gray. When adjusting contrast, remember not just to look after the extremes and the contrast in the image, but keep an eye on the mid tones. Jan is co-author of the suite of Power Retouche Photoshop plugins for photo retouching.
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