Visualization of the effects of the photometric zero-point variation. Panel a): CMDs zoomed into the MS region of NGC 6397 before the zero-point correction; panel b): CMD after the zero-point correction; panel c): CMD after the differential reddening correction (in red the reddening vector, scaled by a factor 1/3); panel d) map of the color zero-point variation; the dashed line divide the two groups of stars shown in the panels a) and b); panel e): final Δmag correction grid for the filter v_HIGH and the NGC 6397 data set.