  We investigate how galaxy-galaxy lensing measurements depend on the knowledge
of redshifts for lens and source galaxies. Galaxy-galaxy lensing allows one to
study dark matter halos of galaxies statistically using weak gravitational
lensing. Redshift information is required to reliably distinguish foreground
lens galaxies from background source galaxies and to convert the measured shear
into constraints on the lens model. Without spectroscopy or multi-colour
information, redshifts can be drawn from independently estimated probability
distributions. The COMBO-17 survey provides redshifts for both lens and source
galaxies. It thus offers the unique possibility to do this investigation with
observational data. We find that it is of great importance to know the
redshifts of individual lens galaxies in order to constrain the properties of
their dark matter halos. Whether the redshifts are derived from $UBVRI$ or the
larger number of filters available in COMBO-17 is not very important. In
contrast, knowledge of individual source redshifts improves the measurements
only very little over the use of statistical source redshift distributions as
long as the source redshift distribution is known accurately.
