Depth of Field

Three factors that affect Depth of Field:

1)  Large (big opening) apertures will render background blurry
     Small apertures render the entrire scene in sharp focus

2) Long telephoto lens settings will render a background blurry
    Wide angle lenses will render the entire scene in sharp focus

3) Close proximity will render the background blurry
    If you're far away from all the subjects in you scene, they will all be rendered in sharp focus

Those photographers who cannot change their aperture can still create a blurry background for portraits or other subject matter by using a close subject with a telephoto lens setting.