2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/9762We obtain analytic formulae for the null geodesics of Friedmann-Lema\^ıtre-Robertson-Walker spacetimes with scalar perturbations in the longitudinal gauge. We use these to provide a rigorous derivation of the cosmological lens equation. We obtain an expression for the magnification of a bundle of light rays in these spacetimes without restriction to static or thin lens scenarios. We show how the usual magnification matrix naturally emerges in the appropriate limits.Submitted to ApJl, 9 pages plus one figure (plain TeX)AstrophysicsGeneral Relativity and Quantum Cosmologytext