ABSTRACT. We have calculated improved photonic passbands for the UBV RI, Hipparcos H p , and Tycho B T and V T standard systems using the extensive spectrophotometric libraries of NGSL and MILES. Using the H p passband, we adjusted the absolute flux levels of stars in the spectrophotometric libraries so that their synthetic H p magnitudes matched the precise Hipparcos Catalogue value. Synthetic photometry based on the renormalized fluxes was compared with the standard UBV RI, B T and V T magnitudes, and revised synthetic zero points were determined. The Hipparcos and Tycho photometry system zero points were also compared with the V -magnitude zero points of the SAAO UBV RI system, the homogenized UBV system, and the Walraven V B system. The confusion in the literature concerning broadband magnitudes, fluxes, passbands, and the choice of appropriate mean wavelengths is detailed and discussed in the Appendix.