Degrees of freedom of the model and the mass-sheet transformation. Values of the product between the third and first derivative of the lens potential, evaluated at the Einstein radius, ψ′ψ″′, as a function of 1 − ψ″, for various lenses with fixed Einstein radius. The star marks the values of a lens generated with the model of section 4.1, assuming log M_* = 11.5, R_e = 7 kpc, log M_DM, 5 = 11.0, γ_DM = 1.5. The pink region covers the range of values spanned by the model, obtained by varying the three model parameters while keeping the Einstein radius fixed. The blue line indicates the range of values obtained by applying the mass-sheet transformation of Equation 4 to the original lens, with values λ ∈ [0.9, 1.1]. The green line corresponds to a power-law lens model.