- shooting with extension means pushing the lens further, i.e. path for light is longer and there is loss of light, so it will be darker.
- shooting macro means shallow dof and any movement even very small ones are greatly magnified, so it is easy to be blurry.
- D90 needs cpu contact to read signal, so non-cpu type extension tube will not transmit information and you will get F--
- G lens can't set aperture manually in this context, so useless unless you shot at f/22 all the time
- not sure if your extension tube allows the lens aperture to open up, please check your 50mm when mounted look through the lens, it is wide open or is the aperture closed? suspect it is open. If it does not have the aperture coupling (open the lens aperture until shooting time then close them for shooting) then you will have full time stopped down situation. The procedure is then focus with the aperture wide open (or nearly wide open) and then stop down the lens to desired aperture to shoot. Brings back memory of the 70's and early 80's.
- MF all the way for your situation.
If you want a simpler way out, get the Kenko extension tube set, it will transmit aperture value, AF (not that it is all that useful in macro), let you set aperture on G lens from the body. About $200 for a set of 3.