Radial magnification ratio between image 1 and 2 for a lens with a power-law density profile, as a function of the power-law index γ. Curves obtained for image configurations with different values of the asymmetry parameter ξ_asymm defined in Eq. (10) are shown. The vertical shaded region indicates the typical uncertainty on the power-law slope, Δγ = 0.05, obtained by modelling high-resolution images of strongly lensed extended sources (Shajib et al. 2021). The horizontal shaded region is the uncertainty on r_μr corresponding to an error on the power-law slope of Δγ = 0.05 in the case of an image asymmetry ξ_asymm = 0.4.