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We discuss the use of Sloan Digital Sky Survey (SDSS) ugriz point-spreadfunction (PSF) photometry for setting the zero points of UBVRI CCD images. Froma comparison with the Landolt (1992) standards and our own photometry we findthat there is a fairly abrupt change in B, V, R, & I zero points around g, r, i~ 14.5, and in the U zero point at u ~ 16. These changes correspond to wherethere is significant interpolation due to saturation in the SDSS PSF fluxes.There also seems to be another, much smaller systematic effect for stars withg, r > 19.5. The latter effect is consistent with a small Malmquist bias.Because of the difficulties with PSF fluxes of brighter stars, we recommendthat comparisons of ugriz and UBVRI photometry should only be made forunsaturated stars with g, r and i in the range 14.5 - 19.5, and u in the range16 - 19.5. We give a prescription for setting the UBVRI zero points for CCDimages, and general equations for transforming from ugriz to UBVRI.

Setting UBVRI Photometric Zero-Points Using Sloan Digital Sky Survey ugriz Magnitudes