Gravitational-lensing critical curve
= Gravitational-lensing critical curve
{title2=$\det A(\boldsymbol\theta)=0$}
A critical curve is a locus in the image plane where the <Jacobian matrix> of the <thin gravitational lens equation> becomes singular. Its formal point-source <lensing magnification> diverges, while finite source size regularizes the measured flux. Its mapped locus in the source plane is a <gravitational-lensing caustic>.