  We measure surface brightness fluctuation (SBF) magnitudes in the F814W
filter and g-I colors for nine bright early-type Fornax cluster galaxies imaged
with the Hubble Space Telescope Advanced Camera for Surveys (ACS). The goal is
to achieve the first systematic SBF calibration for the ACS/F814W bandpass.
Because of its much higher throughput, F814W is more efficient for SBF studies
of distant galaxies than the ACS/F850LP bandpass that has been used to study
nearby systems. Over the color range spanned by the sample galaxies,
1.06<g-I<1.32 (AB mag), the dependence of SBF magnitude mbar_I on g-I is linear
to a good approximation, with slope $\sim2$. When the F850LP SBF distance
measurements from the ACS Fornax Cluster Survey are used to derive absolute
Mbar_I magnitudes, the dependence on g-I becomes extremely tight, with a slope
of $1.8\pm0.2$ and scatter of 0.03 mag. The small observed scatter indicates
both that the estimated random errors are correct, and that the intrinsic
deviations from the SBF-color relation are strongly correlated between the
F814W and F850LP bandpasses, as expected. The agreement with predictions from
stellar population models is good, both in slope and zero point, indicating
that our mean Fornax distance of 20 Mpc is accurate. The models predict
curvature in the relation beyond the color limits of our sample; thus, the
linear calibration should not be extrapolated naively. In the Appendices, we
reconsider the Tonry ground-based and Jensen NICMOS SBF distance catalogues; we
provide a correction formula to ameliorate the small apparent bias in the
former, and the offset needed to make the latter consistent with other SBF
studies. We also tabulate two new SBF distances to galaxies observed in the ACS
Virgo Cluster Survey.
