  Current and future imaging surveys will measure cosmic shear with a
statistical precision that demands a deeper understanding of potential
systematic biases in galaxy shape measurements than has been achieved to date.
We investigate the effects of using the point spread function (PSF) measured
with stars to determine the shape of a galaxy that has a different spectral
energy distribution (SED) than the star. We demonstrate that a wavelength
dependent PSF size, for example as may originate from atmospheric seeing or the
diffraction limit of the primary aperture, can introduce significant shape
measurement biases. This analysis shows that even small wavelength dependencies
in the PSF may introduce biases, and hence that achieving the ultimate
precision for weak lensing from current and future imaging surveys will require
a detailed understanding of the wavelength dependence of the PSF from all
sources, including the CCD sensors.
