Shape parameters against semi-major axis radius as a function of the Einstein radius. Columns, from left to right: SDP 81, SPT 0532−50, and SPT 0538−50. The points show the F110W (blue) and F160W (mauve) isophote fits and their 1σ errors. The yellow curves show the isodensity parameters for PL+MP for the approximate range of radii where there is lensed emission (the convergence 1σ errors are too small to see). The shaded grey region denotes the FWHM of the F160W point spread function (the larger of the two filters). All angle definitions are east of north. The multipole symmetry means that ϕ₃ cycles over 120 deg and ϕ₄ over 90 deg, so jumps of those magnitudes are not physical.