We give the proof of principle of a new experimental method to determine the aberrations of an optical system in the field. The measurement is based on the observation of the intensity point-spread function of the lens. To analyze and interpret the measurement, use is made of an analytical method, the so-called extended Nijboer-Zernike approach. The new method is applicable to lithographic projection lenses, but also to EUV mirror systems or microscopes such as the objective lens of an optical mask inspection tool. Phase retrieval is demonstrated both analytically and experimentally. The extension of the method to the case of a medium-to-large hole sized test object is presented. Theory and experimental results are given. In addition we present the extension to the case of aberrations comprising both phase and amplitude errors. © 2003 Society of Photo-Optical Instrumentation Engineers.