Top panel: fraction of recovered stars after the artificial-star experiment in the i′ observations of NGC 4240. The dotted line is the fitted interpolation function (see Eq. (3)). The dashed blue lines indicate the 50% level of completeness. Middle panel: behavior of the ALLSTAR parameter sharp. Bottom panel: SEXTRACTOR stellarity index. In both panels artificial stars are indicated with black circles, while the real sources are plotted in red. The blue dashed lines indicate the point source criteria applied to the science data based on the artificial one.