Well that's a good pic, don't think it can improved that much, you can see the apeture is at f5.0, and the focal length is 50mm.
Your lens is 18-200mm f/3.5-5.6, which means that at 18mm, the most you can open it is f/3.5. At 200mm the most you can open it, is f5.6.
So I would guess that...