People buy more expensive, larger aperture lenses to get LESS depth of field. A lens "upgrade" is typically going to have a wider aperture, (smaller fstop), to allow in more light and create more blurred backgrounds. The lens you have is just as capable at shooting at f16 as a lens that costs $1000 shooting at f16. There is no lens that is specifically for "larger depth of field". You do that by setting a smaller aperture, (higher fstop number), on the lens. Either your kit lens or the 50mm can be used. I would say you should probably use the 50mm. If you are zooming the kit lens, that will also contribute to a shallow depth of field. So use the 50mm and set an aperture as small as you can, say f11 or f16 to keep your DOF as deep as you can. As was alluded to in another answer, how small you can set the aperture is going to be determined by how much ambient light you have in the scene. In very dim light, you are going to need to open it up, say to f2.8, but in bright conditions, I would think you could use f11 or f16 with no problem, and that will give you the DOF you are seeking.