I recently came across a 10.5/2.8 that a friend is selling for a song. I tried it out and the problem seems to be that the lens is underexposing by around 2 stops, especially when used in matrix mode. Spot and center avg seems to be not as bad but needs a bit of compensation as well. What would normally cause this? It's not the camera because all my other lenses are fine shot at 2.8. Is this easy to fix? Anyone have any idea how much it's going to cost to fix? I can return it if it'd be too much trouble or expense. Thanks!