Note A diverges for images at the Einstein radius \theta_E.
\psi(\vec{\theta}) \approx {1 \over 2} \kappa_{\rm smooth} |\theta|^2 + \sum_i \theta_E^2 \left[ \ln\left( { |\vec{\theta}-\vec{\theta}_i |^2 \over 4} { D_d \over D_{ds} } \right) \right].