  The redshift evolution of bars is an important signpost of the dynamic
maturity of disk galaxies. To characterize the intrinsic evolution safe from
band-shifting effects, it is necessary to gauge how bar properties vary locally
as a function of wavelength. We investigate bar properties in 16 nearby
galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) at
ultraviolet, optical and mid-infrared wavebands. Based on the ellipticity and
position angle profiles from fitting elliptical isophotes to the
two-dimensional light distribution, we find that both bar length and
ellipticity - the latter often used as a proxy for bar strength - increase at
bluer wavebands. Bars are 9% longer in the B-band than at 3.6 um. Their
ellipticity increases typically by 8% in the B-band, with a significant
fraction (>40%) displaying an increase up to 35%. We attribute the increase in
bar length to the presence of star forming knots at the end of bars: these
regions are brighter in bluer bands, stretching the bar signature further out.
The increase in bar ellipticity could be driven by the apparent bulge size: the
bulge is less prominent at bluer bands, allowing for thinner ellipses within
the bar region. Alternatively, it could be due to younger stellar populations
associated to the bar. The resulting effect is that bars appear longer and
thinner at bluer wavebands. This indicates that band-shifting effects are
significant and need to be corrected for high-redshift studies to reliably
gauge any intrinsic evolution of the bar properties with redshift.
