I have a Nikon F3 that is consistently four stops off (underexposed) in manual mode compared to the correct exposure in Aperture priority. Batteries are fresh, aperture index ring moves freely, springs back nicely, and is in the correct position. I did recently mount a non-ai lens on it once on accident without flipping up the tab, and the lens mounted fairly easily (which is why I didn't notice) but was a little more difficult to dismount. Doesn't seem to have caused any damage. Could this be the problem? I do think I remember the meter being OK afterwards, but the timing is pretty close. Any ideas?