Top: Differential cross section as a function of logarithmic lens mass. The magnification threshold is labeled in each panel. The total area under each curve is normalized to unity. Bottom: Differential probability distribution for the optical depth as a function of lens redshift. The optimal lens redshift changes from 0.5 to 1 with changing source size and magnification threshold. Larger source sizes and magnification thresholds cause a lower optimal lensing redshift.