  Recently, weak measurements were used to measure small effects that are
transverse to the propagation direction of a light beam. Here we address the
question whether weak measurements are also useful for measuring small
longitudinal phase shifts. We show that standard interferometry greatly
outperforms weak measurements in a scenario involving a purely real weak value.
However, we also present an interferometric scheme based on a purely imaginary
weak value, combined with a frequency-domain analysis, which may have potential
to outperform standard interferometry by several orders of magnitude.
