A combination detector for detecting a visually identifiable property and an optical property of an optical system. A phase visualization element converts an incoming light bundle to one or more output light bundles in which the spatial phase distribution of the incoming light bundle is visually apparent. The phase visualization element is arranged relative to a detection surface of an image sensor such that the output light bundle or the output light bundles is/are incident only on a first partial region of the detection surface, while a second partial region of the detection surface is exposed in the direction of incidence in order to detect the incoming light bundle which is uninfluenced by the phase visualization element. An apparatus for testing the optical system includes a light source for generating a measuring light bundle and a combination detector as described.