Some extension tubes have contacts so the camera can communicate with the lens ( for circa £50), some are dumb tubes and much cheaper. The OP already has a 50mm AF-D lens, so the aperture could be set on the lens, focus manually and use dumb tubes, provided the D90 will meter in these circumstances. I know my D7000 would meter but my D3200 would not. Perhaps a D90 expert could advise on the D90 metering capabilities, for such an arrangement.