Dodge and burn are techniques that come from the darkroom. When projecting a negative onto photopaper through an enlarger, photographers used to either block light, or cup the hand to direct light into certain areas. This allows underexposing or overexposing of different parts of a photograph. This is the kind of technique that Andsel Adams employed.
Today, we can perform the same function very easily in Photoshop. There are two main reasons to dodge (brighten) and burn (darken).
The first is to fix photos. For example, you may want to open up shadows in a portion of a photo or tone down the highlights to reveal the detail better.
The second is purely creative. As the eye is usually drawn to brighter parts of a photograph, the creative use of dodging and burning can be used to guide the viewers eye to the areas of the photograph that you consider important. Toning down less important areas and enhnacing other areas can simplify the composition and create a very eye catching composition.
You can download some free goodies, get the actions pack and the free ebook at http://photoshopcafe.com/freepresets
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