Illustration of the transformed power-law profile in the mean dimensionless enclosed projected mass distribution under the internal MST with θ_s = 10″. All the transformed mass profiles share the same Einstein radius (red dashed line). All the mass distributions in this figure produce essentially the same model images (i.e., the difference of the χ² is less than 0.001%) but a different unlensed size of the source, which is not directly observable.