Instrument performance for the two channels (I- and R-band; left and right panels respectively). In each panel the measured magnitudes of stars in the open cluster NGC 1960 are plotted against their measured uncertainty (σ_mag) for three different exposure settings (10 s (circles), 1 s (asteriscs) and 0.023 s (crosses)). The curves passing through the photometric measurements are the model predictions of Eq. (1) providing information on the magnitude level that can be reached at a certain σ_mag level (see text for more details). The horizontal line in each panel indicate the 0.1 mag noise level.