The quickest way to see the impact of this strategy is to switch your camera into aperture priority mode and to take a number of shots at different apertures. Start with an aperture of f/20 and work your way down – one stop at a time. Once you get down to under f/4 you’ll start seeing the background in your shots getting blurrier and blurrier.

Photo by Keith Morris

Check out these tips on forming a great relationship between your subject and its background.

Make sure you read the whole thing.

http://digital-photography-school.com/blog/getting-backgrounds-right/