hoping someone can help. i have my beseler 45 mcrx set to the 2 1//4 setting. i'm using a 75 schneider. i have the enlarger mounted on a home made adjust-able and am printing 16x20. i can't seem to get rid of dark corners on the projection even with it wide open. do i have to do something with the actual condenser's or is there something else i can check? thank you.