We are trying to print 4x5 negs on a Beseler enlarger. The problem is the projected image has a very bright center and darkens to the edges. We have set the enlarger to the 4x5 setting, checked the condensers for proper position. No matter what we tweek or turn the result is the same. I don't know the model # but the enlarger has a motorized focus and adjustable voltage control for the light. We are using a 75mm lens. Any insight to what the problem may be will be greatly appreciated.