Painting masks
Once an adjustment layer has a mask, you can modify it by painting on it directly.
Click on the mask of the adjustment layer.
Select the paint brush (B). Make the foreground color pure white to paint on the image, but to change the mask pixels, causing the adjustment effect to be applied to that area of the image. Make it black for the reverse effect.
(Make the foreground color some shade of gray for a partial application of the adjustment effect, or change the brush opacity.)
Zoom in appropriately and notice the edges of your selected area on your adjustment layer. You will see the boundary of the change.
Make your brush an appropriate size and hardness and paint on the image. You should see the affected area change as you paint white or black on the mask.
To see the effect on the mask, hold ALT and click on the mask. You can use this to clean up any missed areas.
