Optical depth as a function of magnification. Different colors correspond to different source sizes. The optical depth for μ ≥ 2 is virtually independent of source size. This is because the region with μ ≥ 2 in the source plane is much larger than the size of the caustics or the source. Hence, the cross section for μ ≥ 2 is dominated by the low‐magnification region, and the optical depth is almost the same for point sources and finite sources.