The present invention relates to limiting an x-ray beam used in connection with dental extra oral imaging by a plate mechanism (1) arranged to be operated by a drive mechanism including an actuator (3) arranged to move at least one plate element (2, 3) comprised in the mechanism (1). The plate mechanism (1) includes at least a first and a second plate element (2, 3) which include at least a first slot (12) and a second slot (13), respectively, and said drive mechanism is arranged to directly or indirectly move said first plate element (2) independently of location of said second plate element (3) and said second plate element (3) is arranged to be moved as dependent on the movements of said first plate element (2) only.