Enlarger fogging

[grainmagnifyer]

Having some darkroom trouble with fogging on two enlargers. Both enlargers are Beseler 35's. These two particular enlargers produce images that are consistently 2-3 stops lighter on the edges. I've adjusted the condenser lenses, played with the negative holder alignments and changed bulbs, even consulted Beseler to no avail. Anyone have remedies, suggestions and/or solutions? Dodging and burning works for advanced students but doesn't help the new ones, and the dept. can't afford new equipment.