When retouching photos one should first look at white balance and contrast. White balance is normally the thing one ought to address 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 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 normal way to manipulate contrast is simply by altering 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 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 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.
What if the darkest and brightest areas are not black and white? In that case one should be able to expand the brightest range to reach black and white. This is essentially what levels adjustment does. 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.
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 normal way to manipulate contrast is simply by altering 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 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 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.
What if the darkest and brightest areas are not black and white? In that case one should be able to expand the brightest range to reach black and white. This is essentially what levels adjustment does. 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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Further information about white balance can be read here: White balance and more about contrast can be read here: Contrast
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