In the good old days, back in the darkrooms with photo paper, there was a black and white system called multigrade. Multigrade paper was available in six grades and each grade produced images with more or less contrast. Today software provides contrast adjustment, but not all contrast adjustments are equal or equally good.
What is contrast anyway? One (noun) definition of contrast could be: "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 photos have three different kinds of contrast: contrast of luminance, contrast of color and contrast of saturation. Most software offers a single contrast slider that edits all three at once.
Software usually alters contrast by simply altering the difference between the middle value (128) and the individual red, green and blue values; 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 one will 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 method, in both cases, is that photos tend to become overly saturated and colorful with contrast enhancement. Similarly contrast reduction makes the picture look like you have placed a semi-transparent gray film over it.
When talking about 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. Photoshop has a checkbox in their contrast editing panel called: "Use Legacy". This method has the drawback that the image becomes pushed towards the primaries R, G and B if you enhance contrast and pushed towards a uniform mid-gray if you reduce contrast. It is not very useful.
Lumiance contrast, however, can be enhanced in several ways. 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 one 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 one wants 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. Don't use Legacy or the picture will tend towards either black and white or uniform gray. When adjusting contrast, remember not just to look at the extremes and the contrast in the image, but keep an eye out for the mid tones.
What is contrast anyway? One (noun) definition of contrast could be: "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 photos have three different kinds of contrast: contrast of luminance, contrast of color and contrast of saturation. Most software offers a single contrast slider that edits all three at once.
Software usually alters contrast by simply altering the difference between the middle value (128) and the individual red, green and blue values; 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 one will 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 method, in both cases, is that photos tend to become overly saturated and colorful with contrast enhancement. Similarly contrast reduction makes the picture look like you have placed a semi-transparent gray film over it.
When talking about 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. Photoshop has a checkbox in their contrast editing panel called: "Use Legacy". This method has the drawback that the image becomes pushed towards the primaries R, G and B if you enhance contrast and pushed towards a uniform mid-gray if you reduce contrast. It is not very useful.
Lumiance contrast, however, can be enhanced in several ways. 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 one 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 one wants 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. Don't use Legacy or the picture will tend towards either black and white or uniform gray. When adjusting contrast, remember not just to look at the extremes and the contrast in the image, but keep an eye out for the mid tones.
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