A fast algorithm is presented which allows for substantially simultaneous acquisition, analysis, and display of a wavefront centroid image, referred to as online aberrometry. A method embodiment involves determination of an average, or most frequently occurring, wavefront aberration over a selected time interval, e.g., 20 sec. Online pupil diameter measurement allows analysis of wavefront aberration as a function of changing pupil size. A wavefront measuring apparatus is disclosed that supports online aberrometry.