Field-to-field variations of number counts in COSMOS. Top panels: number counts split into sub-areas (lines) compared to full-field counts (black dots). Bottom diagram: standard deviation among the sub-fields (top), as a function of flux. In the very bottom panels, we show the comparison between the field-to-field σ(var) and the uncertainty σ_P obtained combining Poisson statistics, photometric errors and systematics. Shaded areas represent 1σ uncertainties on σ(var) and σ(var)/σ_P determinations.