  A general approach to the measurement of an observable with pre- and
post-selection is presented. The limit of weak measurement is studied in
detail, and it is shown that the phase of the probe, including a Hamiltonian
contribution to it, gives rise to observable effects, since the coherence of
the probe is essential for the concept of complex weak value to be meaningful.
As a particular example, the measurement of a spin component is considered. We
find that the contribution of the imaginary part of the weak value is sizeable
in this case.
