  We study the two main corrections generally applied to narrow-band H-alpha
fluxes from galaxies in order to convert them to star formation rates, namely
for NII contamination and for extinction internal to the galaxy. From an
imaging study using narrow-band filters, we find the NII and H-alpha emission
to be differently distributed. In most disk star formation regions the NII
fraction is small, whereas some galaxies display a diffuse central component
which can be dominated by NII emission. We consider explanations for variations
in the NII/H-alpha ratio, including metallicity variations and different
excitation mechanisms. We then estimate the extinction towards star formation
regions in spiral galaxies, firstly using Br-gamma/H-alpha line ratios. We find
that extinction values are larger in galaxy nuclei than in disks, and that
there is no evidence for heavily dust-embedded regions emerging in the near-IR.
We use optical emission line data from the literature to show only a moderate
dependence of extinction on inclination, consistent with broad-band photometric
studies; typical extinctions are smaller for late-type dwarfs than for spiral
types. Finally, we show that the application of type-dependent extinction
corrections significantly improves the agreement between star formation rates
from H-alpha and IRAS fluxes. This agreement argues against star formation in
normal galaxies being dominated by heavily dust-embedded components.
