  I present a semiclassical analysis of a spatial-mode demultiplexing (SPADE)
measurement scheme for far-field incoherent optical imaging under the effects
of diffraction and photon shot noise. Building on previous results that assume
two point sources or the Gaussian point-spread function, I generalize SPADE for
a larger class of point-spread functions and evaluate its errors in estimating
the moments of an arbitrary subdiffraction object. Compared with the limits to
direct imaging set by the Cram\'er-Rao bounds, the results show that SPADE can
offer far superior accuracy in estimating the second and higher-order moments.
