Lily Galib, Production Associate, Image Quality Control, has written a three-part post on the ins-and-outs of light value adjustments. Part 1 covered histograms, part 2 working with Photoshop’s Levels Adjustment Tool and the Curves Adjustment Tool, and part 3 concludes with color.
When making color adjustments with the Levels Adjustment Tool and the Curves Adjustment Tool, the basic functions are the same (3 set adjustment points for Levels; anchor points for Curves) but you break the histogram down into the individual color channels. There are a few different color working spaces, but the ones you will most commonly encounter are RGB and CMYK. Generally speaking, RGB is used for images that will be viewed on a screen and CMYK is used for print. Digital cameras capture images in RGB, which stands for red, green, and blue. CMYK stands for cyan, magenta, yellow, and black, which are the ink colors used in printing. I’m only going to deal with RGB here since it is the color space you’re most likely to use. At ARTstor we always work in RGB.
The complete RGB histogram that you work with when making light value adjustments is actually made up of three individual color channels: a red channel, a green channel and a blue channel. Digital cameras and scanners capture images using sensors which have receptors for red, green, and blue light. When you take a digital photograph, your camera is recording the light values of each of these colors separately and saving them as three separate color channels. These are then sandwiched together to create a complete image. The information recorded is only light values; your imaging software interprets that information and displays it as color.
When you break your histogram down into the individual channels, you’ll see that it looks quite similar to the complete RGB histogram, but instead of 0 representing black and 255 representing white, 255 is the maximum amount of the primary color and 0 is it’s absence, and therefore appears as it’s opposite. In other words, if you look at the red channel, you have 255 representing red and 0 representing the absence of red, which will be cyan. In the green channel 255 is green and 0 is the absence of green, or magenta and in the blue channel 255 is blue and 0 is yellow. When you make adjustments to the individual channels you are changing the amount of the primary color. For example, if you are working in the red channel and you move the adjustment point at 0 to the right, thus making your image more cyan, you are not actually adding cyan but decreasing red. Because the opposite of red is cyan, an absence of red will give your image a cyan cast, and the same goes for green and blue with respect to their opposites.
Curves is the main tool that I use for color corrections. I may use Levels to make some basic color adjustments, but I prefer Curves because, as with light values, it allows for more control and specific adjustments.