Here is something I've learned from beta reading Dan Margulis' Professional Photoshop, 5th edition (due for publication in the fall of 2006). Consider it a teaser.
Steps to tune Shadow/Highlight parameters:
- Set Amount to 100%, Radius to 3%
- Tune Tonal Width to target the adjustment to the relevant areas of the image.
- Tune Radius to bring out the details in that area.
- Tune Amount to make it look good.
Not very different from the steps to tune USM, really.
Now I'll walk though these steps to bring out the highlights using this image by Sid. (Shadows are completely analogous and are left as an exercise for the reader.)