The invention concerns a method and apparatus for limiting a ray beam, especially in connection with an x-ray imaging equipment. The invention comprises a collimator construction in which a primary blade element (4) is moved directly by means of an actuator (3) and where at least one secondary element (5, 20) is movable in connection with the primary element (4), but does not move dependent on the movements of it when the distance between the said elements (4, 5, 20) is within at least one certain operating range concerning them.