Beam deflection units in light-scanning microscopes are usually arranged in planes that are conjugate to the objective pupil. The scan optics, which is required for generating the conjugate pupil planes, is complicated and not very light efficient. The invention is intended to enable a higher image quality, simpler adjustment and a lower light loss microscope. The optical system comprises a concave mirror (36) for imaging a respective point of the first and second beam deflection units (30A, 30B) onto one another. The concave mirror (36), the first beam deflection unit (30A), and the second beam deflection unit (30B) are arranged such that the illumination beam path is reflected exactly once at the concave mirror (36). A first distortion caused by the concave mirror (36) and a second distortion of the imaging caused by the first and second beam deflection units (30A, 30B) at least partly compensate for one another.