In general, the larger the zoom, the better, and the larger the aperture, the better when looking for this effect. There are some nice 3rd party lenses out there that won't quite cost you an arm and a leg. Maybe just an arm. I have seen some x-200mm lenses with a fixed f2.8 for under $1000 that would be a great general purpose lens for this. Prime lenses (non zoom) are also great. You don't have zoom for these lenses, but you will generally get a larger aperture requiring less light for the scene. I am NOT a DSLR expert by any means, and you will probably get more in depth information here, but that is the basics as I understand them. I will be watching this thread though to learn more myself!