When retouching photographs one should first look at white balance and contrast. White balance is normally what you ought to retouch 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 addresses the hue or tone of the light in the photo and normally has white as a goal. White balance apps attempt to correct the hue of the illumination to neutral and in order to do that, the application usually needs some neutrals in the photograph to calculate the correct filter color 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. 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. Software usually has a single slider for contrast adjustment, which addresses all three at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. Two sliders should be the minimum: one for luminance contrast and one for color contrast.
The standard way to manipulate 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 similar for green and blue. This method is only suitable for images that cover the entire brightness range. What about very dark or very pale images? 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 similarly for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.
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. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one's software does not offer the option to expand brightness range, one can do it with Photoshop's levels adjustment like this: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.
White balance addresses the hue or tone of the light in the photo and normally has white as a goal. White balance apps attempt to correct the hue of the illumination to neutral and in order to do that, the application usually needs some neutrals in the photograph to calculate the correct filter color 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. 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. Software usually has a single slider for contrast adjustment, which addresses all three at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. Two sliders should be the minimum: one for luminance contrast and one for color contrast.
The standard way to manipulate 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 similar for green and blue. This method is only suitable for images that cover the entire brightness range. What about very dark or very pale images? 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 similarly for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.
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. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one's software does not offer the option to expand brightness range, one can do it with Photoshop's levels adjustment like this: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.
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More information about white balance can be read here: White balance and more about contrast can be read here: Contrast