Bob Sunley , May 30, 2010; 01:41 a.m.
For a shallower depth of field you want a larger aperture and a longer focal length. A 600mm f4 has a really shallow depth of field, but at $10k, you might be looking for something in a lower price range like a 50mm f1.4. :)
You have to know what focal length range you are looking at to be able to choose a lens to get that shallow depth of field.