The disclosure provides an optical system and a method of characterising an optical system, such as in a microlithographic projection exposure apparatus. According to an aspect, an optical system having an optical axis (OA) includes a first element which is partially translucent for light of a working wavelength of the optical system. The first element has at least one partially reflecting first surface arranged rotated about a first axis of rotation in relation to a plane perpendicular to the optical axis (OA). The optical system also includes a second element in succession to the first element along the optical axis (OA). The second element is partially translucent for light of the working wavelength and has at least one partially reflecting second surface which is arranged rotated about a second axis of rotation in relation to a plane perpendicular to the optical axis (OA). The system further includes an intensity measuring device configured to measure the intensity of light reflected at the first surface and the intensity of light reflected at the second surface.