Histogram of the number of stars (in bins of 1 mag) in an area of 20 × 20 arc-min² towards the SGP from the USNO-B1 catalog that satisfy the criteria that their astrometric position has a positional uncertainty less that a threshold value of σ_crit = 0.1 arcsec. Light gray is for those objects that satisfy the astrometric threshold in the catalog directly (106 objects). Light+dark gray is the number of objects that would satisfy the criteria when one observation with EFOSC2@NTT is carried out on the same sample, according to the expectations from the classical MCR, for a FWHM of 0.7 arcsec. Finally, light+dark gray+black is the number of objects when the BCR is considered (in this case σ_priori is the catalog value for the positional uncertainty of each individual object). The upper panel is for the R band, lower panel for the B band.