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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.

Gravitational lensing probes the cluster's projected mass through the deflection of background light. Strong gravitational lensing produces arcs and multiple images; weak gravitational lensing measures coherent distortions of many background galaxies. The lensing convergence is $\kappa =\Sigma /{\Sigma }_{\mathrm{crit}}$, withThus the image geometry and shear constrain the projected surface density $\Sigma$ without assuming the cluster is in dynamical or hydrostatic equilibrium. The reconstructed total mass again exceeds the visible stellar and gas contribution. Source-distance errors, line-of-sight structures and the mass-sheet degeneracy affect the reconstruction, but the independent agreement of lensing, galaxy motions and gas confinement is strong evidence for cluster dark matter.