Lensing effect on an infinitesimally small source. Locally a source that is circularly symmetric gets lensed into an ellipse. The eigenvectors of the Jacobian of the lensing function y shown in Eq. (3) are parallel to the semiminor and semimajor axes of this ellipse. If the source has unit radius, the lengths of the semiminor and semimajor axes are given by the eigenvalues of the Jacobian.