An optical element (1) of an optical system has at least one chamber (5) that is sealed against atmospheric pressure and is enclosed by boundary surfaces and that has a fluid filling. At least one of the boundary surfaces of the chamber (5) is exposed at least partially to illumination light. It is configured so that a change in the fluid pressure inside the chamber (5) results in a change in non-rotational-symmetric imaging properties of the optical element (1) having n-fold symmetry. For this purpose, a fluid source has a fluid connection to the chamber via a fluid supply line (17). Furthermore, a control device is provided for the pressure in the fluid filling.