A combination of a focusing element, and a filtering element which naturally adjusts the cross-sectional area of a beam of electromagnetic radiation passed through the focusing element as a function of wavelength over a specified range of wavelengths, wherein the filtering element is not uniform, but rather varies as a selection from the group consisting of: optical density and/or thickness is greatest near the center thereof; and optical density and/or thickness is smallest near the center thereof; and can demonstrate neutral density characteristics outside the specified range of wavelengths. The combination of a focusing element, and a filtering element can optionally be present in an ellipsometer or polarimeter system.