1. Open a back-lit photo with underexposed subject. Or use my sample image for this exercise.

2. From Adjustments, open Levels (Remember Lesson Two?). Under the histogram, move the middle slider until the faces looks just right. You might also have to move the highlight slider (on the right) a bit. Yeah, I know the background gets too light. We'll fix that later. Choose OK.

3. Bring up the History panel (you may have to open it from the Window pull-down). The History panel remembers back, way back, farther back than any of us want to know. But right now you've only made one change, so you should see only two options: "Open" (original photo) and "Levels" (after your change). Click on "Open." Original photo returns.

4. Still working from the History panel, click on that box to the left of the Levels state (now grayed out). The History Brush icon jumps to attention. You're going to be painting history, mon.

5. From the tool box choose the History Brush (brush with arrow). Try a smallish soft-edged brush size from the choose bar at top. Still in the original photo (Open) state in the History Palette, paint in the faces. Avoid painting in the background. Sometimes photographers say they "paint with light," but this is the real thing, digally speaking.