I''ve been using a D100 for some time. A few days ago I noticed the image in the viewfinder has gotten very dark. I"ve checked all the settings to no avail; reset it to factory defaults etc. Its the same with both the lenses I have. If i manually change the f setting the image brightens up with the lower f numbers otherwise if set to the correct mode it stays dark. Any ideas anyone?

Do you have a manual depth-of-field preview button?
Sounds like that is stuck on.

Or does the lens have a m/a aperture setting?
One of my tamron lenses when set to A would operate wide open until the camera stopped it down, but set to M would follow the aperture settings on the ring.

I would first look to see if it's something that's got knocked and turned on/off that you haven't normally used.