The image shown above was taken with a 70-200 mm lens with a 2x teleconverter at an effective focal lenght of 380mm. An Aperture of f8 was chosen to insure the face and body of the bird appeared sharp and to provide a pleasing background.

Depth of field, or the portions of the image that appears sharp, is a function of distance, focal length, and Aperture. Distance and focal length also determines the size of the subject in the frame, leaving the aperture as the determing factor for how sharp or blurred the background appears. I like the look of this images as the background appears as out of focus spots of color, drawing attention to the sharp part of the image, the bird. There is no doubt that it is the subject.

This image was taken with my 600mm lens with a 1.4X Teleconverter. It was taken with the Aperture set wide open at f5.6. In this case with the extreme focal length and wider Aperture, as well as the greater distance to the background, the background is completely blurred. Again the out of focus background draws attention to the bird.

In both of the cases the choice of lens used was due to the subject being photographed. In many cases, like when you're photographing people, you can choose to use a larger focal length lens with a wide aperture and move further from the subject giving you the creative control to draw attention to the subject. Such as this photograph of my Grand Daughter taken with a 200mm lens at f2.8