= Mass-sheet degeneracy
{title2=$\kappa\mapsto\lambda\kappa+(1-\lambda),\quad\gamma\mapsto\lambda\gamma$}
For a single source plane, adding a uniform sheet and rescaling the original projected lens mass as shown leaves the reduced shear $g=\gamma/(1-\kappa)$ unchanged. Rescaling source coordinates by $\lambda$ also preserves the image positions and shapes. Thus image geometry alone need not determine absolute lens mass normalization. Absolute magnification, independently known source sizes, time-delay information with a fixed distance scale, or additional geometric/dynamical constraints can break or restrict this <mass-sheet degeneracy>.
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