Left panels: photometric uncertainties due to the quadratic propagation of the errors for each star. Right panels: uncertainties computed for stars detected in the overlapping regions between fields using Eq. (1). The uncertainties shown in the right panels reach lower magnitudes since those stars need to be detected in more than one field, and the slightly different atmospheric conditions between pointings complicate the detection of the faintest stars. Only a random fraction of the stars has been plotted for clarity.