Dodging and Burning in the Digital Darkroom
Dodging and burning are among the oldest and most time honored techniques in the traditional darkroom. By selectively blocking or focusing the light while printing the negative, the operator could compensate for the limited range both of film and the printing medium, toning down highlights and opening up blocked-up shadows. While the way we dodge and burn has changed in the digital darkroom, the techniques remain as helpful as ever.
Recognizing the usefulness of dodging and burning, Adobe built tools for them into Photoshop. To prevent the palette bar from becoming even more crowded than it already is, a number of locations actually contain more than one tool. The Dodge Tool, Burn Tool, and something called the Sponge Tool all share one such location. By clicking on whichever one is visible and holding the mouse button down, you will get a fly-out menu that will let you reach the others.
Leaving the sponge aside for today, the other two icons a relevant to the task at hand but may require a bit of explaining. The thing that looks like an oversized map pin is really supposed to be a dodging tool. Traditional darkroom operators used to hold various shapes made of metal or other opaque materials over the print on the end of long wires in order to temporarily block the light. Except for a few positive processes such as Ilfochrome (Cibachrome), prints are made from negatives, so they get darker as more light falls on them. The positive image gets formed because darker portions of the negative block more light than lighter portions do. An area of a print could be made lighter (dodged) by selectively blocking more light than the negative already did. The darkroom operator could use their hands (the other icon) to block larger areas or focus the light only on selective areas to darken them (burning).
The technique in Photoshop is roughly the same. In order to lighten, select the Dodge Tool. To darken, use the Burn Tool. If you want a way to keep straight which is which and can't remember how traditional printers did this in the darkroom, just visualize that burning things turns them black. This fact isn't technically relevant since we won't be burning anything in that sense, but it can make a good mnemonic device for remembering that burning makes things darker. Next, choose a brush that will allow you to paint the area of the image you want to modify. You can decide to modify Highlights, Midtones or Shadows by means of the Range drop-down box. The Exposure slider will let you control the strength of effect. Be careful not to be too overzealous with your changes since the Burn and Dodge tools actually modify data in your image and can not be undone except by means of the History palette.
If you're uncomfortable with the idea of making edits to your actual image, there is an alternative method that lets you to burn and dodge on a layer instead. It is a bit more complicated, but not by very much. It does not make use of the Burn and Dodge tools at all.
The first thing you'll need for this technique is a new layer. Rather than just any layer though, it needs to be a special kind of layer. From the menus, select Layer >> New >> Layer .... You will be presented with a dialog box allowing you to specify the options for your new layer. As an alternative, you can get the same dialog by clicking on the "new layer" icon at the bottom of the Layers palette while holding down the Alt/Option key. Regardless of how you get there, select "Overlay" in the Mode drop-down. A box labeled "Fill with Overlay-neutral color (50% gray)" that will appear underneath it. Check it and click on "OK." Your new layer will show up in the Layers palette as gray but will not affect the appearance of your image at all.
In Overlay mode, anything darker than 50% gray will darken the underlying layer, while anything brighter than that will lighten it. We can use this fact to selectively burn and dodge an image. Press "D" to reset your foreground and background colors to their default white and black colors. Then, select an appropriately sized brush and adjust its Opacity to around 10% and paint away. You can press the "X" key to swap foreground and background colors and toggle which one you are painting with. If you overdo it, you can switch back by using the opposite color. Be sure you are painting on the Overlay layer rather than your actual image of course.
While Levels and Curves let you brighten or darken an entire image and make other global adjustments to tone and contrast, dodging and burning let you make localized changes. It won't let you make a bad image good, but it will help you get the most out of your good images by optimizing their tonality.
Update 7/04/2005 - As an alternative to the Overlay blending mode, you may want to try "Soft Light" mode for even more subtle effects. Of course, you can always tone down the effect by adjusting the opacity of the burn and dodge layer in either blending mode.