Using selective focus to show a subject in sharp focus with other elements in the scene blurred is a wonderful technique for isolating your subject against an abstract background. Use a normal or telephoto lens at a wide aperture setting to produce shallo w depth of field. It subdues objects in the background or foreground that would otherwise steal attention from your subject. The type of lens that you use greatly affects the depth of field at any aperture setting.
Most people buy telephoto lenses because they make distant subjects appear larger in the film frame.
However, these lenses are also great for selective-focus techniques. The longer the focal length, the greater the blurred-background effect. And the closer the subject is to the camera, the shallower the depth of field. When you use selective focus for portraits, always focus on your subject's eyes, the most important feature.