Comparative modeling of a simulated ring in the four cases considered in Sect. 3. Top panels: mock observations of the lens system under CFHT (gri-color, left) and HST/WFPC2 (gray-scale, right) conditions. Other panels: images where the deflector has been subtracted ideally (middle panels) or using galfit (bottom panels), along with the lensed features reconstructed by the model and the critical lines (in red). The CFHT simulations are modeled in the g band. Each panel is on a side. Although the original lensing configuration is not perfectly recovered for a galfit subtraction of the deflector from the mock ground-based observation, the brightest parts of the ring are still identified, allowing a robust estimation of R_Ein. In this example we find (for an initial value ), and the reduced χ²/ν ~ 3.4 per degree of freedom.