An adjustment mechanism for adjusting an axial position of one lens component relative to other lens components of an optical lens system comprises a lens holding frame mounting the lens component that has a peripheral locating seat and a cylinder holding and allowing the lens holding frame to rotate about and move along the optical axis that has an axial position selective collar with a flight of stairs. The lens holding frame is rotated to bring the locating seat into abutment selectively against the stairs, so as thereby to position the lens component mounted in the lens holding frame at a desired axial n with respect to the remaining lens components.