Optical assemblies have one or more optical elements mounted in the cavity of a barrel. The optical element has a curved abutment surface engaging a seat provided in the cavity. The barrel has an inner wall provided with a set of internal threads. External threads are provided on a peripheral wall of the optical element or on a perimeter wall of a retaining ring securing the optical element within the cavity. The internal and external threads have a thread profile including a load-bearing thread face perpendicular to the center axis of the cavity. The thread profile may define a Buttress thread profile or a square thread profile.