Left panel: contribution to the optical depth for a source at redshift z_s = 2 as a function of the redshift of the lens z_l and for all lens masses. Right panel: contribution to the optical depth for a source at redshift z_s = 2 as a function of the mass of the lens M_l and for all lens redshifts. In both cases, the numbers by each curve indicate the magnification that the curve contributes to. For high magnifications, only the most massive halos contribute to the magnification.