  We compare Stetson's photometric standards with measurements listed in a
standard star catalog constructed using repeated SDSS imaging observations. The
SDSS catalog includes over 700,000 candidate standard stars from the equatorial
stripe 82 (|Dec|<1.266 deg) in the RA range 20h 34' to 4h 00', and with the $r$
band magnitudes in the range 14--21. The distributions of measurements for
individual sources demonstrate that the SDSS photometric pipeline correctly
estimates random photometric errors, which are below 0.01 mag for stars
brighter than (19.5, 20.5, 20.5, 20, 18.5) in ugriz, respectively (about twice
as good as for individual SDSS runs). We derive mean photometric
transformations between the SDSS gri and the BVRI system using 1165 Stetson
stars found in the equatorial stripe 82, and then study the spatial variation
of the difference in zeropoints between the two catalogs. Using third order
polynomials to describe the color terms, we find that photometric measurements
for main-sequence stars can be transformed between the two systems with
systematic errors smaller than a few millimagnitudes. The spatial variation of
photometric zeropoints in the two catalogs typically does not exceed 0.01
magnitude. Consequently, the SDSS Standard Star Catalog for Stripe 82 can be
used to calibrate new data in both the SDSS ugriz and the BVRI systems with a
similar accuracy.
