  One of the principal goals of modern cosmology is to constrain the properties
of dark energy. At present, numerous surveys are aiming to determine the
equation of state, which is assumed constant due to our poor understanding of
its behaviour (and since higher order parameterisations lead to unwieldy
errors). This raises the question - how does our "best-fit" equation of state
relate to the true function, which may vary with redshift. Saini et al. have
demonstrated that the value of w attained by a supernovae study is well
described by a weighted integral over the true function. Adopting a similar
approach, we calculate the corresponding "weight function" for a fiducial
cosmic shear survey. We disentangle contributions from the angular diameter
distance and the growth factor, finding that they partially cancel each other
out. Consequentially, the sensitivity to w at high redshift is enhanced beyond
that of supernovae studies, and is comparable, if not higher, than lensing of
the CMB. This illustrates the comlementary nature of the different techniques.
Furthermore, we find that results which would naively be interpreted as
inconsistent could arise from a time-dependent equation of state.
