  We address the relation between quantum metrological resolution and
coherence. We examine this dependence in two manners: we develop a quantum
Wiener-Kintchine theorem for a suitable model of quantum ruler, and we compute
the Fisher information. The two methods have the virtue of including both the
contributions of probe and measurement on an equal footing. We illustrate this
approach with several examples of linear and nonlinear metrology. Finally, we
optimize resolution regarding coherence as a finite resource.
