When editing pictures you should first consider white balance and contrast. White balance is normally what you ought to fix first, then contrast.White balance should be addressed before contrast because color contrast can not be set properly if the image has a color cast.
White balance addresses the color of the illumination within the photograph and sets white as a goal. White balance applications attempt to correct the color of the illumination to white and to do that, the application needs some neutrals in the picture to calibrate the suitable correction tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are more difficult to find in real life, so one can use 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. Apart from the 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 programs can dispense with that, 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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The usual way to manipulate contrast is simply by changing 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 the green and blue channel. If the image is very dark or very pale this method will not do. 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.
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: 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 addresses the color of the illumination within the photograph and sets white as a goal. White balance applications attempt to correct the color of the illumination to white and to do that, the application needs some neutrals in the picture to calibrate the suitable correction tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are more difficult to find in real life, so one can use 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. Apart from the 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 programs can dispense with that, 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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The usual way to manipulate contrast is simply by changing 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 the green and blue channel. If the image is very dark or very pale this method will not do. 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.
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: 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.
