This optical assembly comprises a securing support (40), an optical focusing element (44) and a reflecting mirror (42) which is suitable for reflecting an incident light beam (F1) towards the optical focusing element (44) in order to focus it onto an illumination plane (P). The reflecting mirror (42) is movable relative to the securing support (40). The optical focusing element (44) is movable relative to the securing support (40). The assembly includes elements for connecting the focusing element (44) to the mirror (42) for synchronised displacement of the two of them. The assembly is useful in devices for illuminating operating fields.