The Rosetta spacecraft carried a Navigation Camera (NavCam) for optical navigation in the vicinity of the comet. We performed a cross-calibration study based on images taken with the OSIRIS near-angle camera. For this purpose, we selected sets of images acquired roughly simultaneously on 2014 August 1 during comet approach at small phase angles. We employed two procedures, the first one based on the average signal over the nucleus and the second considering histograms of signal values within the images. Both methods delivered consistent results for the radiometric calibration factor.