How-To
Most photographs can be improved with global adjustments. But quite often, local darkening and lightening tweaks can make the photo better. If you use the dodge and burn tools in Photoshop on the background layer, the changes are permanent. Use this Photoshop Tip to dodge and burn on a separate layer and not embed the changes until you flatten the image. There is no degradation of pixels.
Original Photo: 
Create a new layer onto which all dodging and burning will occur. Option Click on a Mac or Alt Click on a PC on the create a new layer icon at the bottom of the layers palette.
When the dialog box appears, change the Mode to Overlay and enable the Fill with 50% Gray check box.
All dodging and burning will be performed with the brush tool. To lighten the dark parts of the photo, set the foreground color to white. Paint over the dark areas with the brush. To darken the bright parts of the photo, set the foreground color to black. Paint over the light areas with the brush.
Before you paint, be sure the settings in the Options bar resemble this screen capture. Keep the opacity around 20% and make multiple passes to build up the dodge or burn effect. If the effect is too strong, lower the opacity. If it’s not enough, raise it.
Strategy map for areas to lighten:
Strategy map for areas to darken:
Once painted, the layer mask shows the lightened and darkened areas. The light gray areas were dodged. The dark gray areas were burned.
The beauty of this technique is the layer can be revisited and tweaked at any time without degrading any pixels. This is the final image based on the painted layer mask above:
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