Joint posterior distribution of a subset of lens mass parameters, for the 3 main model families tested in this work. The first one includes only external shear in addition of the three main deflectors, the second one replaces external shear with the El Gordo cluster deflection field, and the last one includes a combination of the two. The parameters shown in the figure are the Einstein radii (θ_E,L1, θ_E,L2, θ_E,L3), the logarithmic density slopes at θ_E (γ_L1, γ_L2), the mass axis ratio (q_m,L1), and the external shear orientation and strength (ф_ext, γ_ext, when included in the model). See Fig. C.2 for the full mass model parameters space.