Auto-centering is for an optical element mounted in a cavity of the barrel. A first surface of the optical element engages a seat provided in the cavity. A retaining ring is threaded on the barrel, through complementary barrel and ring threads. The retaining ring engages a peripheral region of a second surface of the optical element, thereby securing the optical element between the seat and the retaining ring. The profile of the barrel threads and the spatial profile of the peripheral region of the second surface are selected in view of an auto-centering condition whereby any decentering of the retaining ring and a corresponding tilt of the retaining ring have counterbalancing effects on the centering of the optical element. Optical assemblies and a mounting method are used with the optical element.