Comparison of the relative errors in the inferred properties, defined as δp/p = (p^fit − p^true)/p^true, for a kinematic model fitted either directly in the image frame (left column) or in the source frame using the GalPaK^3D Strong Lensing Extension (right column). Each row shows the error density distribution for a given property, restricted to the rotation-dominated subsample of mock galaxies (v_1.8/σ₀ > 1). The effective radius is corrected a posteriori by the magnification when fitted in the image plane. The PA error is relative to PA^true = 130°, as in Bouché et al. (2015b). Dashed lines show the empirical relation $\delta p/p=\pm S/N_{\max }\times {(\sqrt{\mu }{R}_e/R_{\mathrm{PSF}})}^{\alpha }$ based on the coefficients α from Bouché et al. (2015b), while solid vertical lines indicate $S/N_{\max }\times {(\sqrt{\mu }{R}_e/R_{\mathrm{PSF}})}^{\alpha }=10$.