Optical assemblies include a barrel defining a cavity having a center axis, a sleeve inserted in the cavity, one or more optical elements mounted within the sleeve and a retaining ring inserted into the cavity and securing the sleeve. The sleeve engages the barrel inner wall through a thread engagement allowing a longitudinal displacement of the sleeve within the cavity. The retaining ring is also threadably engaged within the barrel, and the profile of the corresponding threads, as well as the spatial profile of a peripheral transversal surface of the sleeve engaging the retaining ring, are selected to provide centering of the sleeve with respect to the center axis of the cavity throughout the longitudinal displacement of the sleeve.