The last stop on the Color Correction Express is Curves, the most powerful—and fear-inducing—adjustment in all of Photoshop. The basic idea is that, by curving a diagonal line on a grid, you change the brightness of the pixels in an image. Instead of the three main adjustment sliders you get with Levels (shadows, highlights, and midtones), Curves gives you up to 16 adjustments. But that’s not as scary as it sounds. If you survived the section on Levels relatively unscathed, you already know a ton about Curves. For example:
You can use Curves as an Adjustment layer so that it’s nondestructive, which means you can also use the included layer mask to restrict the adjustment to certain areas of an image. The Curves grid shows up in the Properties panel, just like Levels.
A Curves adjustment uses a histogram and the same 256 shades of gray you saw in Levels. It also has the same shadows and highlights sliders (though no midtones slider), and it harbors the same trio of eyedroppers for resetting the black, white, and midtone points (The Levels Eyedroppers). So far so good!
You can Option-drag (Alt-drag on a PC) the shadows and highlights sliders to find the darkest and lightest areas of your image, like you learned on Making Colors Pop.
You can use Curves to correct an image using the Info panel and the Color Sampler tool, and you can type target values into the Properties panel’s Input field. To summon the Input field (shown in Figure 10-20), click a point on the curve ...