Okay, anything above F8 comes out black (and F8 is pretty dark) no matter what I set the ISO to. When I put this on automatic the ISO goes to a double digit number and the F-stop drops to 5.8 or something like that. On the darker pictures any attempt to lighten them just reveals noise which pretty much destroys and sharp edges.
Stop changing the ISO and adjust the shutter speed. Set the ISO to 400 or 800 and leave it there. Then while looking thru the viewfinder adjust the shutter speed until the metering shows about -1 underexposure in the viewfinder. It will be a very long exposure (1+ sec probably) which is why it's vital to shoot from a tripod and and have no subject motion.