Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
This is a control you’re not going to need to use every time (well, at least hopefully not. If you do, we have bigger problems to discuss). You’ll generally only use the Fill Light slider in situations where your subject is in the shadows, like when your subject is backlit (and you should have used a fill flash when you took the shot).
Step One:
When you open a photo where your subject is in the shadows, the Fill Light slider can do a remarkable job of fixing your problem. My photo assistant, Brad, took this shot of me at Mt. Norquay in Banff, Canada. It was shot late in the day and if you look at the before image on the next page, you can see that I’m backlit. While you can see some detail, the detail areas of the subject (me) are mostly in the shadows. If you drag the Exposure slider to the right (as shown here), it overexposes the sky.