Abstract: The present invention refers to an optical system consisting of an objective lens, an aperture stop, a fixed lens and a movable lens group, whereby the aperture stop corresponds to the primary focal plane of the fixed lens group so that the pupil through which the light beam emerges is formed at the infinite distance. The movable lens group consists of two lenses whose focal planes correspond to each other and even if the movable lens group is moved along, the direction of the optical axis for focusing the position and the dimension of the pupil of the optical system do not change.