It's hard to say much conclusively without more information. But based on what you have provided, my first guess is that something in your lens has gone wrong and the aperture is staying stopped all the way down, regardless of the aperture setting selected by you or by the camera in 'Auto' exposure mode. The well-defined dust spots in your example photo indicate a fairly narrow aperture was used for that image.

You can test this by looking through the front of the lens when it is attached to the camera. The aperture should be all the way open. If it is all the way closed like when the lens is not attached to your camera, that's your problem.

You can also test the lens by taking some test shots and comparing the results.

• First, take a few shots at ISO 100, 1/1000 second, and f/3.5 (or whatever your lens' widest aperture - lowest f-number - is)
• Then, take a few shots at ISO 100, 1/1000 second, and f/22 (or whatever your lens' narrowest aperture - highest f-number - is)

If you are in bright daylight, the first shot should be overexposed by about one to two stops. The second shot should be underexposed by about two to three stops. If they look exactly the same, your lens' aperture is not opening up.

If the aperture is not opening up correctly, it could be an issue with the lens, or it could be an issue with the camera's aperture lever not properly engaging the mechanical connection for it on the lens.

Another test would be to use another Nikon lens on your camera or your lens on another Nikon camera. This will allow you to determine if the issue is with the lens or the camera.

Yes, I did try with a different lens and had the same result. There were slight image differences when I was changing the aperture settings, but nothing close to correct exposure.

That points to your camera's aperture control lever as the likely culprit. It has probably been bent in such a way that it is stopping the lens down much further than it should for the aperture setting selected by the camera. The reason that there is not much difference between the widest and narrowest aperture is that the lens is stopped all the way down at almost all aperture settings. At the widest aperture settings, the lens is still stopped almost all the way down.

This is a known issue with the stop down lever on Nikon F mount cameras. If the lever is bent, it will not stop down accurately with any lens. If the problem is only demonstrated with one lens, the issue is probably in the lens' internal aperture linkage/mechanism.

If you use an ISO and shutter time appropriate for f/22 (or whatever the smallest aperture/highest f-number for your lens is), are the images still underexposed? In bright sunlight that should be something like ISO 200 and 1/100 second.