Step 4

If you look closely, you’ll see the edge of the firework layer where the pixels weren’t as black as possible. To fix it, bring up the Levels adjustment (command/control-L), and slide the black triangle under the histogram to the right until the firework looks great. (What you’re doing is changing the black point to where everything to the left of the black triangle is a pure, dark black– perfect for the Screen mode to hide.)