  We present evolutionary synthesis models of starbursts on top of old stellar
populations to investigate in detailed time evolution the relation between Ha
luminosity and star formation rate (SFR). The models show that several effects
have an impact on the ratio between L(Ha) and SFR. Metallicity different from
solar abundance, a time delay between star formation and maximum Ha-luminosity,
and a varying stellar initial mass function give rise to strong variations in
the ratio of Ha luminosity to SFR and can cause large errors in the
determination of the SFR when employing well-known calibrations. When studying
star-bursting dwarf galaxies, and sub-galactic fragments at high redshift,
which show SFR fluctuating on short timescales, these effects can add up to
errors of two orders of magnitude compared with the calibrations. To accurately
determine the true current SFR additional data in combination with models for
the spectral energy distribution are needed.
