Lensing shear
= Lensing shear
{title2=$\gamma_1=(\psi_{,11}-\psi_{,22})/2,\quad\gamma_2=\psi_{,12}$}
The trace-free part of the angular lensing-potential Hessian stretches an image anisotropically. The source-image <Jacobian matrix> has eigenvalues $1-\kappa\pm|\gamma|$. Thus a circular source becomes an ellipse in the local linear approximation; the orientation follows the shear axes.