An optical system has a set of inner portions, each for mounting over an LED or LED arrangement, and an outer portion, for mounting over the inner portion. The outer portion is adapted to be mountable over each one of the set of inner portions, thereby to form an output lens when the outer portion is mounted over a selected one of the set of inner portions. The output lens has an overall optical function which depends on the selected inner portion. In this way, an optical lens function may be selected simply by choosing an appropriate one from a set of inner portions (or by not using an inner portion at all). The inner portions are low cost components, and they avoid the need for any tooling changes to the larger outer portions.