There's a realationship to the lens, aperture size, and focal length. There are great articles on the subject, but there's nothing like actual footage to make the point. I've been able to get good DOF on the kit 18-135mm lens also, just not as extreme as this example. Shooting this shallow is extremely difficult with moving subjects and makes you have to not only be good with your focus reflexes but also makes you have to decide if you want their nose, eyes, or ears in focus (choose one)!