The lens has a constant f value; I'm guessing f/8. That means that you can only adjust your shutter speed or the ISO setting. So slow the shutter speed or increase the ISO to get the right exposure.. You will most likely need to always use a tripod with this lens and limit it to shots of landscapes, etc. Moving things, like birds and cars, etc. will likely not be good subjects with this lens.
I have a D80 camera and bought a Bower 500MM preset telephoto lens. All my shots come out so dark you cannot define them. What am I doing wrong? What settings am I missing?