Abstract
A more accurate formulation of the Abbe theory is presented using well-known open resonator results, namely, field representation in terms of the eigenfunctions of the optical system viewed as a segment of an equivalent lens waveguide; and the use of the mirror Fresnel number dependence of the diffraction losses of high-order Gaussian beams for describing the image distortions due to the objective aperture. The number of the degrees of freedom of the image within the zero mode waist area is estimated by using the Hermite-Gaussian functions within the paraxial approximation framework.