Results of the measurement of r_eff/r_eff,true by GALFIT and the linear regression method, as a function of the size of the fitting area, for various S/N. The simulated galaxy is a Sérsic profile with n = 3 and a half-light radius of 10 pixels. The left-hand panels show the result from GALFIT, and the right-hand panel from the linear regression method. The top panels correspond to S/N = 800, the middle panels, S/N = 170, and the bottom panels, S/N = 50. The vertical axis is the measured r_eff/r_eff,true. The horizontal line represents r_eff/r_eff,true = 1. The horizontal axis shows the size of the fitting region in units of r_eff,true, ranging from 1 r_eff,true to 5 r_eff,true. The stars are the results for n = 4, and the crosses for the correct value n = 3. Because they are smaller than the symbol size, the σ_rand error bars are not shown in order not to impair the readability of this figure.