More, in line with the above:
add a http://www.bhphotovideo.com/c/product/674566-REG/Kenko_PRO3001_4XDGXN_Teleplus_PRO_300_DG.html
for an effective 14-22mm/4 combination on an FX camera.
Actually the Kenko TC appears to be a way to mount any DX lens on a FX camera and have decent performance out of the combination.
You get a 1.4x boost and a 1 stop loss on aperture.