Comparison of the Petrosian magnitudes of stars (solid circles) after their photometry in the GROND J,H,Ks bands is calibrated with that in the corresponding 2MASS bands (panels a) − c), respectively). Saturated stars and stars with poor photometry were flagged out. The asterisks indicate that the reference magnitudes are expressed in the GROND filter system. In each panel the red solid line represents equality.