When editing photographs one should first look at white balance and contrast. White balance is normally the thing you ought to fix first, 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 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. 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. One can also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. Some software can dispense with that, 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 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 need to change the algorithm 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 algorithms are essentially the same since a full brightness range image will have 128 as an average value.
If the darkest and brightest areas are not black and white a different situation arises. 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: First convert the image to Lab mode, select the L channel only and run auto levels on that. 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. 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. One can also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. Some software can dispense with that, 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 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 need to change the algorithm 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 algorithms are essentially the same since a full brightness range image will have 128 as an average value.
If the darkest and brightest areas are not black and white a different situation arises. 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: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.
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