Mass-sheet degeneracy ($\kappa \mapsto \lambda \kappa +(1-\lambda ),\gamma \mapsto \lambda \gamma$)

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.