Panel a): CMD of NGC 2808; the red arrow indicates the direction of reddening. The continuous lines are the axes ( and ) of the reference frame introduced during the procedure for the measurements of reddening variations. The location of stars in this reference frame is shown in panel b), where we draw the fiducial line of the MS as a dashed red line. Reference stars are indicated with black dots and are located between the dotted lines. The rectified vs. diagram is plotted in the panel c).