I have used this in Photoshop 6. I do not have earlier versions to know whether it will work in them. Credit for this tip must go to Michael Reichmann.

The term "contrast mask" was not familiar to me when Michael mentioned it in Issue 2 of his Luminous Landscape Video Journal. I checked it out and found a tutorial on his site. The technique offers a very simple solution to bring out the information in the darker areas of an image. This is a problem that I had been struggling with for some while - level, curves and selection masks had not produced results with which I was happy.

You can read the full article, but the technique is very simple:

1. Create a new layer
2. De-saturate the layer
3. Invert the layer
4. Change the layer to "overlay"
5. Apply Gaussian blur of between 10 and 50 to suit taste

Optionally, you can adjust the opacity of the contrast mask layer and you can have more than one contrast mask layer for additional control and adjustment.

Update: Michael has written a new article, Understanding Contrast Masking.