  We analyse the functional derivative of the cosmic-shear power spectrum
$C_\ell^\gamma$ with respect to the cosmic expansion function. Our interest in
doing so is two-fold: (i) In view of attempts to detect minor changes of the
cosmic expansion function which may be due to a possibly time-dependent
dark-energy density, we wish to know how sensitive the weak-lensing power
spectrum is to changes in the expansion function. (ii) In view of recent
empirical determinations of the cosmic expansion function from distance
measurements, independent of specific cosmological models, we wish to find out
how uncertainties in the expansion function translate to uncertainties in the
cosmic-shear power spectrum. We find the following answers: Relative changes of
the expansion function are amplified by the cosmic-shear power spectrum by a
factor $\approx 2-6$, weakly depending on the scale factor where the change is
applied, and the current uncertainty of one example for an empirically
determined expansion function translates to a relative uncertainty of the
cosmic-shear power spectrum of $\approx10\,\%$.
