DW, you could use whatever lens you'd like. However, you'd have to focus manually if you don't buy a lens or have a body that is capable of auto focusing. If you do not want to focus manually (on the Nikon D5100), then you need an AF-S lens. The D5100 does not have an in-camera motor that does the focusing, so you need a lens that has a motor inside of it (assuming you want auto focus capability).