  We establish a simple method to assess the quantum Fisher information
required for resolving two incoherent point sources with an imaging system. The
resulting Cram\'er-Rao bound shows that the standard Rayleigh limit can be
surpassed by suitable coherent measurements. We explicitly find these optimal
strategies and present a realization for Gaussian and slit apertures. This
involves a projection onto the optimal bases that is accomplished with digital
holographic techniques and is compared with a CCD position measurement. Our
experimental results unequivocally confirm unprecedented sub-Rayleigh
precision.
