An optical system is described for merging a first and a second partial image beam emanating from a specimen into a resultant image beam allowing modification of the areal proportion of the respective first or second partial image beam in the resultant image beam. A stop arrangement comprises at least a first and a second movable stop element each comprising at least one stop region adapted to be brought into a working position with the first or second partial image beam. Movably arranged connecting means for connecting the two stop elements are provided to modify the respective areal proportions of the partial image beams in the resultant image beam by movement of the connecting means.