Selective Focus is briefly discussed both in Frosty Sumacs and in Tip 20 - Depth of Field Preview, but here are some more details.
As the name implies, the idea is to focus on "selected" objects while leaving others out of focus. Most often the technique is used with the focus on a foreground object with the background out of focus. I find this to be particularly useful when there is a distracting background. However, don't let that stop you from experimenting. The technique emphasizes the object in focus and takes attention away from the objects that are out of focus.
The way to apply the technique is to use a shallow depth of field by using a large aperature (f-stop) on your lens. Remember that the smaller the f-number (such as f2.8), the larger the opening is when compared to a larger f-number (such as f16). If you have trouble visualizing this, detach a lens from the camera body and look through it while you change the aperature. Or, think of the f-stop as a fraction: 1/f2.8 is greater than 1/f16.
Also remember that depth of field is affected by the focal length of the lens being used. A wide angle lens has a much greater depth of field than a telephoto lens at the same aperature. For example, take the following two lenses, a 28mm and a 200mm lens for a 35mm camera system; both are focused on an object at 10 feet with the lens set to f2.8. The 28mm lens will have a depth of field (focus) of over 8 feet (2.5 meters), while the 200mm lens will have a depth of field (focus) of only 1.6 inches (4 cm) !! You can also affect the depth of field by using an extension tube to move the lens farther from the focal plane of the camera and increase image magnification.
Visualize your final image and allow just enough depth of field to acieve your goal. If you are experimenting, try changing the aperature to see what affect it has on the final image. See which one you like best.
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