Imaging systems are typically partitioned into three components: focusing of incident light, scattering of incident light by an object and imaging of scattered light. We present a model of high Numerical Aperture (NA) imaging systems which differs from prior models as it treats each of the three components of the imaging system rigorously. It is well known that when high NA lenses are used the imaging system must be treated with vectorial analysis. This in turn requires that the scattering of light by the object be calculated rigorously according to Maxwell's equations. Maxwell's equations are solvable analytically for only a small class of scattering objects necessitating the use of rigorous numerical methods for the general case. Finally, rigorous vectorial diffraction theory and focusing theory are combined to calculate the image of the scattered light. We demonstrate the usefulness of the model through examples.