  The Diffuse Ionized Gas (DIG) contributes to the nebular emission of
galaxies, resulting in emission line flux ratios that can be significantly
different from those produced by HII regions. Comparing the emission of
[SII]6717,31 between pointed observations of HII regions in nearby galaxies and
integrated spectra of more distant galaxies, it has been recently claimed that
the DIG can also deeply affect the emission of bright, star-forming galaxies,
and that a large correction must be applied to observed line ratios to recover
the genuine contribution from HII regions. Here we show instead that the effect
of DIG on the integrated spectra of star-forming galaxies is lower than assumed
in previous work. Indeed, aperture effects on the spectroscopy of nearby HII
regions are largely responsible for the observed difference: when spectra of
local HII regions are extracted using large enough apertures while still
avoiding the DIG, the observed line ratios are the same as in more distant
galaxies. This result is highly relevant for the use of strong-line methods to
measure metallicity.
