  Great uncertainty surrounds dark energy, both in terms of its physics, and
the choice of methods by which the problem should be addressed. Here we
quantify the redshift sensitivities offered by different techniques. We focus
on the three methods most adept at constraining w, namely supernovae, cosmic
shear, and baryon oscillations. For each we provide insight into the family of
w(z) models which are permitted for a particular constraint on either w=w0 or
w=w0+wa(1-a). Our results are in the form of "weight functions", which describe
the fitted model parameters as a weighted average over the true functional
form. For example, we find the recent best-fit from the Supernovae Legacy
Survey (w=-1.023) corresponds to the average value of w(z) over the range
0<z<0.4. Whilst there is a strong dependence on the choice of priors, each
cosmological probe displays distinctive characteristics in their redshift
sensitivities. In the case of proposed future surveys, a SNAP-like supernova
survey probes a mean redshift of z ~ 0.3, with baryon oscillations and cosmic
shear at z ~ 0.6. If we consider the evolution of w, sensitivities shift to
slightly higher redshift. Finally, we find that the weight functions may be
expressed as a weighted average of the popular "principal components".
