By Karen Hanson
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 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. 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 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+RAve
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+RAve
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.
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.