Optical system (1) : with a first optical subsystem (3) comprising at least a first birefringent optical element (7), with a second optical subsystem (5) comprising at least a second birefringent optical element (9), wherein between the first optical subsystem and the second optical subsystem an optical retarding system (13) with at least a first optical retarding element (15) is arranged, which introduces a ratardation of one-half of a wavelength between two mutually orthogonal states of polarization.