An optical system for arranging between a light source and a field diaphragm in an illumination beam path of a microscope comprises n imaging optical elements with focal lengths fi and Abbe numbers νi (i=1, . . . , n). The following relationship is met for the optical system: ∑ i = 1 n ⁢ h i f i · v i ≤ 0.07 , where hi is one half of the bundle diameter of a bundle of light rays proceeding from a point of the light source at the entrance to the imaging optical element i.