An ellipsometer comprising: a source that emits radiation; a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample; a polarization state analyzer positioned to receive reflected radiation from the sample; a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer; and a focusing system positioned to receive a portion of the reflected radiation that includes an entire cross-section of the reflected radiation, the focusing system comprising: a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation; a lens system that receives the portion of the reflected radiation and magnifies by at least 2x any deviation from a best focus position of the ellipsometer; and a two-dimensional sensor, the lens system focusing the entire cross-section of the reflected radiation into a spot at a location on the two-dimensional sensor, wherein the spot is 50 percent or smaller than the two-dimensional sensor and the location of the spot on the two-dimensional sensor provides an indication with the at least 2x magnification of any deviation from the best focus position, the two-dimensional sensor generating an image signal of the spot; a processor (130, 156, 157) that receives the image signal, filters noise from the image signal and that is configured to find the a location of the spot on the two-dimensional sensor after noise is filtered to determine the deviation from the best focus position of the ellipsometer based on the location of the spot on the two-dimensional sensor. a source that emits radiation; a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample; a polarization state analyzer positioned to receive reflected radiation from the sample; a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer; and a focusing system positioned to receive a portion of the reflected radiation that includes an entire cross-section of the reflected radiation, the focusing system comprising: a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation; a lens system that receives the portion of the reflected radiation and magnifies by at least 2x any deviation from a best focus position of the ellipsometer; and a two-dimensional sensor, the lens system focusing the entire cross-section of the reflected radiation into a spot at a location on the two-dimensional sensor, wherein the spot is 50 percent or smaller than the two-dimensional sensor and the location of the spot on the two-dimensional sensor provides an indication with the at least 2x magnification of any deviation from the best focus position, the two-dimensional sensor generating an image signal of the spot; a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide the portion of the reflected radiation; a lens system that receives the portion of the reflected radiation and magnifies by at least 2x any deviation from a best focus position of the ellipsometer; and a two-dimensional sensor, the lens system focusing the entire cross-section of the reflected radiation into a spot at a location on the two-dimensional sensor, wherein the spot is 50 percent or smaller than the two-dimensional sensor and the location of the spot on the two-dimensional sensor provides an indication with the at least 2x magnification of any deviation from the best focus position, the two-dimensional sensor generating an image signal of the spot; a processor (130, 156, 157) that receives the image signal, filters noise from the image signal and that is configured to find the a location of the spot on the two-dimensional sensor after noise is filtered to determine the deviation from the best focus position of the ellipsometer based on the location of the spot on the two-dimensional sensor.