The invention relates to a method and device for operating collimator (1) for limiting a beam of high-energy radiation (2) which, starting from an essentially point-shaped radiation source (3), is directed onto an object (4) to be treated and which is used especially for stereotactic, conformal radiation therapy of tumors, wherein the collimator (1) has an iris diaphragm (5) as a beam-limiting means. For such a collimator (1), a high degree of shielding for minimal overall height and with a variable opening size of the diaphragm opening (12) is achieved, in that the iris diaphragm (5) has at least three diaphragm leaves (6, 6′, 6″, or 7, 7′, 7″, 7′″, or 8, 8′, 8″, 8′″, 8″″, or 9, 9′, 9″, 9′″, 9″″, 9′″″) which have touching side surfaces (10) enclosing the same angle (α), wherein the diaphragm leaves (6, 6′, 6″, or 7, 7′, 7″, 7′″, or 8, 8′, 8″, 8′″, 8″″, or 9, 9′, 9″, 9′″, 9″″, 9′″″) open up a beam-limiting opening (12) such that a sliding movement (13) along the side surfaces (10) takes place by a number of diaphragm leaves (6, 6′, 6″, or 7, 7′, 7″, 7′″, or 8, 8′, 8″, 8′″, 8″″, or 9, 9′, 9″, 9′″, 9″″, 9′″″) which is reduced by at most one.
