An example of the results of calibration to the SDSS r-band for the NGC 2683 field. Top: SDSS r magnitude r_SDSS of the calibrating stars in the image compared to the corresponding instrumental measurement L_inst; the colour code is (g − r). The red, green, and blue solid lines are the r_cal from Eq. (1) evaluated at the maximum (reddest), median, and minimum (bluest) of the (g − r) values of the sample, respectively. Bottom: magnitude residuals after calibration, Δr = r_SDSS − r_cal, vs. (g − r); the colour code is r_SDSS. No dependency on (g − r) remains. The standard deviation of the residuals in this particular case is 0.05 mag and we reach similar accuracy in calibration of the rest of images (see Table 2).