Luminance and its relationship to color

- In Photoshop and other graphics programs, brightness goes by many names including shade, tone, and my favorite, luminants. In your everyday average RGB image which is far and away the most common kind of image you'll encounter, luminants is measured in steps known as levels. A level of zero is black and a level of 255 is white. The other levels, 1 through 254, represent the progressively brighter shades of grey in between. So where does color come in? Again, assuming an RGB image, you have three luminants-only versions of an image, called channels.

The three channels are respectively colorized red, green, and blue (hence RGB) and then blended together to produce the full color composite. In other words, color is the function of luminants. Meanwhile, luminants in colors sometimes react with each other in surprising ways. Look at a strip of grey values and you see a steady progression of neutral levels, just as you would expect. Throw a cool color like blue into the mix and the blues go from light to dark as expected. But add a warm color like red and we go from cool pinks at the top to a surprisingly intense scarlet at the bottom.

As we progress into the oranges, notice how the most vivid colors move up the list. We eventually arrive at yellow which darkens into an unappetizing shade of green. In other words, changing the brightness of a photo can be tricky. The purpose of this chapter is to show you how to adjust luminants in ways that will always make your images look better and never worse. In the end, I think you'll be amazed at the degree to which you can rescue even the most washed out or murky photograph.