I wrote a tutorial and an action set this weekend for making tone-based masks. You can use the masks to isolate elements in an image based on their luminosity.

I find the technique helpful for restoring midtones to high contrast images. It has many other uses, also. Anytime you want to focus on just a portion of the tonal curve of an image, such as the midtones or the quartertones, you can use this technique to make your layer masks.

The PS action set makes 24 different tonal masks.

Tutorial:

<http://www.thelightsright.com/DigitalDarkroom/Tutorials.htm>

PS Action Set:

<http://www.thelightsright.com/DigitalDarkroom/PhotoshopTools.htm>

Cheers,

Mitch