https://www.creativelive.com/class/working-with-curves-in-photoshop-cc-jesus-ramirez

The Curves adjustment in Adobe® Photoshop® is one of the most powerful tools to adjust color and tonal values in an image. Unfortunately, it can also be one of the most difficult to use. In this class, Jesus Ramirez will take an in-depth look at how curves work, and how you can use it enhance your photos. This class is made up of multiple step-by-step examples that take you through every major feature of the Curves adjustment.