  We show that the lensing efficiency of cosmic shear generically has a simple
shape, even in the case of a tomographic survey with badly behaved photometric
redshifts. We argue that source distributions for cosmic shear can therefore be
more effectively parametrised in ``efficiency space''. Using realistic
simulations, we find that the true lensing efficiency of a current cosmic shear
survey without disconnected outliers in the redshift distributions can be
described to per cent accuracy with only two parameters, and the approach
straightforwardly generalises to other parametric forms and surveys. The cosmic
shear signal is thus largely insensitive to the details of the source
distributions, and the features that matter can be summarised by a small number
of suitable efficiency parameters. For the simulated survey, we show that prior
knowledge at the 10% level, which is attainable e.g. from photometric
redshifts, is enough to marginalise over the efficiency parameters without
severely affecting the constraints on the cosmology parameters $\Omega_m$ and
$\sigma_8$.
