The invention relates to a method of calculating an optical system (OS), the optical system (OS) being identified by a function (OF), the optical system (OS) comprising a first part (F1) defined by a first equation (EF1) and a second part (F2) defined by a second equation (EF2), the method comprising: —a generating step (GEN), in which a virtual optical system (VOS) is used to generate a virtual function (VOF); —a modification step (MOD), in which the virtual function (VOF) is modified so as obtain the function (OF); —a calculation step (CAL), in which the second equation (EF2) is calculated from the function (OF), and the first equation (EF1). The invention relates also to a method of manufacturing an optical system (OS).