Lensing convergence
= Lensing convergence
{title2=$\kappa=\tfrac12\nabla_\theta\cdot\boldsymbol\alpha$}
The trace of the angular derivative of the reduced deflection defines convergence in the standard <thin gravitational lens equation> convention. It equals projected <surface mass density> divided by the critical surface density, $\Sigma_{\rm crit}=c^2D_s/(4\pi GD_dD_{ds})$. If deflection is instead defined as source minus image angle, the divergence relation has a minus sign.