  Upcoming weak lensing surveys on wide fields will provide the opportunity to
reconstruct the structure along the line of sight tomographically by employing
photometric redshift information about the source distribution. We define
power-spectrum statistics, including cross correlation between redshift bins,
quantify the improvement that redshift information can make in cosmological
parameter estimation, and discuss ways to optimize the redshift binning. We
find that within the adiabatic cold dark matter class of models, crude
tomography using two or three redshift bins is sufficient to extract most of
the information and improve the measurements of cosmological parameters that
determine the growth rate of structure by up to an order of magnitude.
