Sounds like your missing the movable tray condensor. If so, you should use the 150 lens (intended for the 4x5 format with the moving condensor removed) with your medium format neg.s intead of your 80. This will cure the light falloff issue, but you're going to have raise the head to greater heights for an equivalent size print than you would with the proper condensor/lens set. The light will be less intense as well, but not a huge issue.