Some examples to get one started using layer masks

Our first exercise is to turn a colour shot into very selective colour against a monochrome image.
To begin, drag the Background thumbnail over the "Create a new layer" icon to create a duplicate layer.   Next click on the "Add layer mask" icon which will add a blank layer mask.
This is the Layer Palette with the mask in place. Click on the image beside it to select it, then desaturate it.   Once desaturated, click again on the mask thumbnail. The Tool Palette swatches should show as black and white. Using the brush tool paint over the area you want in colour with black.
As you paint on the image window, you will see black accumulating on the mask, as the colour comes through from the background layer. Black = Transparent / White = Opaque. Should you paint beyond where you want the colour to come through, switch to white and paint with opacity to restore it. Simple as that! Choose Flatten from the Layer menu and the job is done.
A similar technique can be used to reduce the effect of skin imperfections while retaining a feeling of sharpness.
Once again duplicate the background layer by dragging over the "Create a new layer" icon.   Under the Filter Menu, select Blur->Lens Blur and apply it liberally to the new layer. Then add a layer mask as before.
Again, paint with black to let the details you want to be sharp come through. In this case eyes and lips. Remember that you can always paint with white to correct any overpainting.   Finally use the slider above the mask to set the balance between the soft image and the sharp image below.
In this example, we are in a situation where we want to avoid blowing out highlights, even though the bulk of the detail is in deep shadows. The exposure was purposefully set to keep all the highlight detail.

By now, it is probably second nature to duplicate the background layer by dragging the layer thumbnail to the "Create a new layer" icon.

DO NOT click on the Layer Mask icon yet!

  Go to the selection menu and choose "Highlights". Surprisingly, this will select all the highlights in your image. These are the parts of the image you want to stay intact, while opening up the shadows. When the highlights are selected, expand them slightly using Selection->Modify->Expand. I used 5 pixels. Next, invert the selection by choosing Inverse from the Selection menu. Feather the selection using double the value that you chose for expanding it. Select->Feather.
NOW click on the Layer Mask icon, and notice that it AUTOMATICALLY created a mask allowing the highlights to show through.   Finally, change the layer mode from Normal to Screen. Suddenly your shadows will light up. If they light up too much, use the slider again to lower them to taste. If they don't light up enough, flatten the image and run the routine again. In this case I ran it twice, and backed off with the slider on the second run.

You now have the basis to continue to explore this immensely powerful feature.

©2006 Larry N.Bolch