Lensing shear (${\gamma }_1=({\psi }_{,11}-{\psi }_{,22})/2,{\gamma }_2={\psi }_{,12}$)

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.