Rigorous approach to gravitational lensing

We revisit a weak gravitational lensing problem by constructing a setup which describes the actual system as accurately as possible and solving the null geodesic equations. Details are given for the case of a universe driven only by a cosmological constant, Lambda, which confirm the conventional results: The conventional lensing analysis is correct as it is, without any need for correction of O(Lambda). We also treat the cases of the lensing in generic Friedmann-Robertson-Walker backgrounds.