When retouching pictures you should first address white balance and contrast. White balance is usually what you ought to consider first, then contrast.This order is important, because you can not set color contrast properly if the image has a color cast.
White balance is concerned with the hue or tone of the light within the picture and normally has white as a goal. White balance software attempts to correct the hue of the illumination to white and in order to do that, the app normally needs some whites or grays in the image to calibrate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW photos, one normally has a 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. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. There are a few programs that can dispense with the neutrals, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.
The standard way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise 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 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. In that case one should be able to expand the brightest range to reach black and white. Levels adjustment is meant for this type of correction. 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 is concerned with the hue or tone of the light within the picture and normally has white as a goal. White balance software attempts to correct the hue of the illumination to white and in order to do that, the app normally needs some whites or grays in the image to calibrate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW photos, one normally has a 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. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. There are a few programs that can dispense with the neutrals, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.
The standard way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise 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 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. In that case one should be able to expand the brightest range to reach black and white. Levels adjustment is meant for this type of correction. 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.