Comparison of mass profiles displaying twists or ellipticity variations with a mass profile following a pure ellipsoid. Left: example of input profiles created with the slice method as explained in Sect. 2.1, displaying either twists (top) or ellipticity changes (bottom). Middle: singular isothermal ellipsoid with the ellipticity and position angle of the corresponding sliced profile at the Einstein radius, i.e., 1″. Right: difference of the two convergence maps in percentage of the input profile. The dashed region indicates the region inside the Einstein radius.