  (Abridged) We use 101 galaxies selected from the Nearby Field Galaxy Survey
(NFGS) to investigate the effect of aperture size on the star formation rate,
metallicity and reddening determinations for galaxies. We compare the star
formation rate, metallicity and reddening derived from nuclear spectra to those
derived from integrated spectra. For apertures capturing <20% of the B(26)
light, the differences between nuclear and global metallicity, extinction and
star formation rate are substantial. We calculate an `expected' star formation
rate using our nuclear spectra and apply the commonly-used aperture correction
method. The expected star formation rate overestimates the global value for
early type spirals, with large scatter for all Hubble types, particularly late
types. The differences between the expected and global star formation rates
probably result from the assumption that the distributions of the emission-line
gas and the continuum are identical. We discuss the implications of these
results for metallicity-luminosity relations and star formation history studies
based on fiber spectra. To reduce systematic and random errors from aperture
effects, we recommend selecting samples with fibers that capture >20% of the
galaxy light. For the Sloan Digital Sky Survey and the 2dFGRS, redshifts z>0.04
and z>0.06 are required, respectively, to ensure a covering fraction >20% for
galaxies similar to the average size, type, and luminosity observed in our
sample. Higher luminosity samples and samples containing many late-type
galaxies require a larger minimum redshift to ensure that >20% of the galaxy
light is enclosed by the fiber.
