Mass function determination. Left panel: G_RP vs. G_BP − G_RP CMD of NGC 2099. The magnitude intervals used to derive the MF are highlighted with different colors, while the median mass of each bin is indicated on the right. Right panel: MF for the stars plotted in the left panel, i.e., log(N/ΔM) vs. stellar mass. The blue and azure straight lines represent the best-fit straight lines of low- (M ≤ 1 M_⊙) and high-mass stars (M > 1 M_⊙), respectively. The red line is obtained by fitting the whole MF. The slopes are quoted in the bottom-left legend, while the total stellar mass of the cluster is indicated in the top-right inset.