I just bought a black K-x with the kit lens. I am looking for a secondary lens with the primary aim of being able to take shallow depth of field images. It could be any third party lens too. As I won't be using this lens much, what would be the most cost-effective lens to buy. Do I need to buy at least a 2.8 lens or will a higher zoom, but not as wide an aperture do. Image quality shoudl be reasonable not necessarily top-of-the-line