That's the infinite plane wave approximation for lattices of infinite extent. Scattered spherical waves from finite objects will result in some energy passing through the aperture for every spatial frequency. Although it could be difficult to sort out which frequencies are contributing (aliasing). Analysis of the through focal series can do that, also changing the convergence of incident illumination.
But if the source is known to be two points, accurate measurement of the spacing between the resulting PSFs is limited only by signal to noise.