Two Photoshop tools, Levels and Curves, let you address almost all tonal issues, and mastering them is a basic necessity for productive Photoshop work. As you'll find out later in this book, Levels and Curves aren't always necessarily the easiest way to correct tone, but until you've learned what they can do, you just don't know Photoshop.
However, two additional Photoshop features, the Histogram and Info palettes, provide information that can help guide your corrections. They let you analyze both the unedited image and the effect of your tonal manipulations. So before plunging into the Levels and Curves dialog boxes, both of which are quite deep, let's look at the analytical tools: the Histogram palette and the Info palette.