actually,,you can have some fun,,, by turning off the DX mode, and you can use DX lens in FX mode.. So... why ?  well,, you can use a sigma 10-20 lens,, in FX mode..
At 10 mm,,  you will see a circle,, but,,,, at 14 mm or so,, the circle covers the whole FX sensor!  
So,, your DX lens becomes a 15-20 FX wide angle !  how cool is that?