Resolving power is defined as the the ability of an imaging device to separate or to make to see as distinct points of an object that are located at a small angular distance.

There is a relationship between diffraction and aberration which can be can be characterised by the point spread function or PSF. The narrower the aperture of a lens the more likely the PSF is dominated by diffraction. In that case, the angular resolution of an optical system can be estimated from the diameter of the aperture and the wavelength of the light. This is what is explained by the Rayleigh criterion.