Here is another note to myself but readers are welcome. For a long time I have not been able to master the use of Mask in Photoshop. Today I managed to pick it up after following a tip in a photography magazine.
Here is one technique that uses Mask to darken the sky while keeping the foreground constant. First, open the file in Photoshop. Make a duplicate (drag the "background" layer to the "create a new layer" icon). Use Lasso tool to get a rough outline of the sky area that needs to be darkened. Go to Select>Colour Range; and while holding the shift key click on different parts of the sky until all the black in the preview area disappears. Next, click "add layer mask" icon to turn the selection into mask. Now grab the Blend mode pulldown menu and switch to Multiply. Finally, to fine tune the area where the sky meets the horizon, use Gaussian Blur.
Variations of this technique include:
1. Instead of the Blend option, use Levels to change the brightness of the sky.
2. Instead of using the Lasso tool, use one of the Channels to aid in selecting the target area. Remember to also use Levels or Contrast to aid in isolating the target area.
Hooray! I feel I have crossed another milestone in Photoshop!