An optical imaging system generates an image of an object plane and includes a lens system which, in turn, includes a main objective and a reduction optical unit ahead of the main objective. The lens system is aligned along an optical axis and the reduction optical unit includes a first lens with a positive refractive power and a second lens with a negative refractive power. An object-side first main plane and an image-side second main plane are defined by the lens system. The optical imaging system defines an observation beam path which is guided through the lens system so that, in the first main plane and in the second main plane, the observation beam path is at a distance (B) from the optical axis. The first lens is of a first material having a first Abbe number and the second lens is of a second material having a second Abbe number, wherein the first Abbe number is greater than the second Abbe number.