By Michael Henriksen
When retouching photographs one should first look at white balance and contrast. White balance is usually the thing one should fix 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 concerns the hue or tone of the light in the image and sets white as a goal. White balance applications attempt to retouch the hue of the light to white and to do that, the software normally needs some neutrals in the image to find the suitable correction tint 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 usually has both a manual and an automatic mode. 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. There are a few Photoshop plugins that can dispense with the neutrals, but usually neutrals are needed
There are three kinds of contrast: hue, saturation and brightness. Normally software only has a single slider for contrast that addresses all three aspects at once. A single slider usually results in an over saturated image and gaudy colors. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The usual way to adjust contrast is simply by changing 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 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.
What if the darkest and brightest areas are not 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: 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 concerns the hue or tone of the light in the image and sets white as a goal. White balance applications attempt to retouch the hue of the light to white and to do that, the software normally needs some neutrals in the image to find the suitable correction tint 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 usually has both a manual and an automatic mode. 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. There are a few Photoshop plugins that can dispense with the neutrals, but usually neutrals are needed
There are three kinds of contrast: hue, saturation and brightness. Normally software only has a single slider for contrast that addresses all three aspects at once. A single slider usually results in an over saturated image and gaudy colors. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The usual way to adjust contrast is simply by changing 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 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.
What if the darkest and brightest areas are not 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: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.