When retouching images one should first address white balance and contrast. White balance is normally the thing one ought to consider to begin with, then contrast.White balance should be addressed before contrast because color contrast can not be set properly if the image has a colorcast.
White balance deals with the hue or tone of the light within the image and normally has white as a goal. White balance apps attempt to adjust the color of the illumination to neutral and to do that, the app normally needs some whites or grays in the photo to find the suitable correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. 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.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Software usually has a single slider for contrast adjustment, which addresses all three at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. Luminance contrast and color contrast should be treated separately.
The usual way to manipulate contrast is simply by altering the difference between the individual R, G and B values and the average value (128); like this: R= (R-128) * contrast + 128; and similar for the green and blue channel. This method is not suitable for very dark or very pale images. 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 deals with the hue or tone of the light within the image and normally has white as a goal. White balance apps attempt to adjust the color of the illumination to neutral and to do that, the app normally needs some whites or grays in the photo to find the suitable correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. 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.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Software usually has a single slider for contrast adjustment, which addresses all three at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. Luminance contrast and color contrast should be treated separately.
The usual way to manipulate contrast is simply by altering the difference between the individual R, G and B values and the average value (128); like this: R= (R-128) * contrast + 128; and similar for the green and blue channel. This method is not suitable for very dark or very pale images. 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.
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