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There he was, right in front of me. The King of Trees - or something like that. Bedecked in foliage with his face and hands painted to match, he cut an impressive figure. But how could I get the shot?
You see, I was at a Renaissance Faire, and this character had the most impressive costume I'd seen yet. But the background included cars and people in modern dress. They'd spoil the mood of my shot.
Fortunately, I understand selective focus... and the shot was saved. The King came out in sharp focus, while the distracting background elements were a pleasant blur.
How can you do the same? It's easy.
First, get comfortable using your camera's manual settings. Then you can use a wide aperture to narrow your depth of field. This little trick lets you decide how much of what's in front of your lens will be sharp. (This is one reason higher-end digital SLR's have a depth-of-field preview button.)
Second, you can use a telephoto setting or long lens. The longer the focal length setting of your lens, the narrower the depth of field. When you're at a greater distance from your subject, "pulling them closer" with your zoom will result in a narrower depth of field.
Either way, the narrower depth of field will allow you to blur elements that might distract from your image.