Method for limiting an X-ray beam, wherein the X-ray beam is limited by a limiting unit comprising a couple of blades approaching each other or moving away from each other to adjust the width of a space between the blades the two blades being driven by an actuator (12) linked to both blades. The position of a symmetry axis (S) of the space between the blades is set in a shifting step by changing the distance between the blades and by impeding the motion of one blade in comparison with the motion of the other blade resulting in an asymmetric motion of the blades with respect to an initial position of the symmetry axis (S). The desired width (W) of the space between the blades is set in an adjustment step by an unimpeded symmetric motion of the blades with respect to the position of the symmetry axis (S) set in the shifting step.