In the framework of a long-term program aimed at studying stellar populations in the Carina dSph galaxy, we present the photometric calibration of the two mosaic cameras adopted in this experiment, the MOSAICII CCD camera available at the 4-m CTIO Blanco telescope, and the WFI available at the 2.2-m ESO/MPI telescope. We discuss the data reduction strategy we devised for the the absolute calibration of individual chips in the two instruments. In particular, we investigated the occurrence of positional effects, which affect the magnitude estimates according to the position of the star in the individual chips. The use of a large sample (several hundreds) of standard stars per chip indicates that the MOSAICII camera is marginally affected by positional errors. The WFI@2.2-m presents a positional error when moving from the center to the edge of individual chips. We present analytical relations for B and V passbands.