Average differences between PSF and aperture magnitude of the tertiary stars in g_Mr_Mi_Mz_M bands as a function of the star magnitude (once corrected for the colour dependent terms). The back error bars represent the statistical uncertainty on the average. The shaded areas represent the uncertainty on aperture magnitudes due to a systematic uncertainty on the residual background in the images used for calibration (0.1 ADU per pixel for g_Mr_Mz_M, 0.2 ADU for the i_M-band).