  We investigate the amount of scatter in the Tully-Fisher relation (TFR) when
using optical long-slit H-alpha rotation curves to determine the velocity
widths of spiral galaxies. We study a sample of 25 galaxies in the Coma region
of the sky which were shown in Bernstein et al. (1994) to exhibit an
extraordinarily low scatter of 0.10 mag RMS in the I magnitude vs 21-cm width
TFR. Using the same I magnitudes with new widths derived from high-quality
H-alpha rotation curves, we measure an RMS scatter of 0.14 mag in the TFR. This
suggests that measurement errors and ``astrophysical errors'' (such as
non-circular gas motion) on the H-alpha velocity widths are below 6%, and
optical widths are nearly as good for TFR studies as 21-cm widths. The scatter
and form of the TFR are found to be robust under choice of velocity
width-extraction algorithm, as long as the central portions of the optical
rotation curve are ignored and low-S/N points are not weighted too heavily. In
this small sample there is no evidence that rotation curve shapes vary
systematically with rotation velocity, nor that rotation curve shape can be
used to reduce the scatter in the TFR.
