  I use distance- and mass-limited subsamples of the Spitzer Survey of Stellar
Structure in Galaxies (S4G) to investigate how the presence of bars in spiral
galaxies depends on mass, colour, and gas content and whether large, SDSS-based
investigations of bar frequencies agree with local data. Bar frequency reaches
a maximum of $\approx 0.70$ at $M_{\star} \sim 10^{9.7} M_{\odot}$, declining
to both lower and higher masses. It is roughly constant over a wide range of
colours ($g - r \approx 0.1$-0.8) and atomic gas fractions ($\log (M_{HI} /
M_{\star}) \approx -2.5$ to 1). Bars are thus as common in blue, gas-rich
galaxies are they are in red, gas-poor galaxies. This is in sharp contrast to
many SDSS-based studies of $z \sim 0.01$-0.1 galaxies, which report f_bar
increasing strongly to higher masses (from $M_{\star} \sim 10^{10}$ to $10^{11}
M_{\odot}$), redder colours, and lower gas fractions. The contradiction can be
explained if SDSS-based studies preferentially miss bars in, and underestimate
the bar fraction for, lower-mass (bluer, gas-rich) galaxies due to poor spatial
resolution and the correlation between bar size and stellar mass. Simulations
of SDSS-style observations using the S4G galaxies as a parent sample, and
assuming that bars below a threshold angular size of twice the PSF FWHM cannot
be identified, successfully reproduce typical SDSS f_bar trends for stellar
mass and gas mass ratio. Similar considerations may affect high-redshift
studies, especially if bars grow in length over cosmic time; simulations
suggest that high-redshift bar fractions may thus be systematically
underestimated.
