  We present deep Hubble Space Telescope optical imaging observations of two
Galactic globular clusters, 47 Tuc and M15, using the Space Telescope Imaging
Spectrograph (STIS) longpass (LP) filter and the Wide Field and Planetary
Camera 2 (WFPC2) F606W and F814W filters. These globular clusters have very
different metallicities ([Fe/H] = -0.7 dex for 47 Tuc, and [Fe/H] = -2.2 dex
for M15), essentially spanning the metallicity distribution of the entire Milky
Way globular cluster system, and were chosen to investigate the relationship
between magnitudes in the non-standard STIS LP system and the better
characterized WFPC2 magnitudes. We determine this relationship by combining our
new STIS data with archival WFPC2 data, taking care to provide a robust and
reliable transformation. Examining the 47 Tuc and M15 data separately, there is
no evidence for a metallicity dependence in the transformation relations. This
allows us to combine the data for both clusters, and also include the stars in
the Small Magellanic Cloud that lie within the field of view of the 47 Tuc
images, to determine final transformations that are valid for main sequence
stars with colors in the range 0.3 < V606-I814 < 1.8. These final relations
predict STIS magnitudes within 0.05 mag (1 sigma) of those measured for stars
in both clusters. In an appendix, we discuss the differences between our
empirical transformation relations and those predicted by synthetic photometry.
