You can change the opacity of a layer, brush, or eraser by using the keyboard, which helps you compare different opacities on the fly.

To change the opacity of a…

1. Brush: Select the brush tool (keyboard shortcut: B), then type your number
2. Eraser: Select the eraser tool (keyboard shortcut: E), then type your number
3. Layer or Folder: Make sure you don’t have a brush or eraser selected (I usually hit “V” on the keyboard to select the move tool), then type your number.

Change the opacity to…

1. 10: Hit 1 on the keyboard. Same goes for 20 (2), 30 (3), etc.
2. 31: Hit 31 (quickly) on the keyboard. Same goes for 42 (42), 74 (74), 9 (09), etc.
3. 100: Hit 0 on the keyboard.
4. 0: Why would you do that? Just hide or delete your layer!