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 between the main objective and the object plane. The reduction optical unit includes a first composite element with a positive refractive power and a second composite element with a negative refractive power. The first composite element includes a first and second lens. The second composite element includes a third and fourth lens. 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 each of the first main plane and the second main plane, the observation beam path is at a distance from the optical axis of the lens system.