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A ray and wavefront analysis is the basis of an interpretation of the aberration balancing merit function in Grey's lens design program. From each of several field angles, this program traces a coarse grid of rays through an axially symmetric optical system. A polynomial in a region around each ray describes the wave error. This polynomial includes terms (through second order) in aperture, field angle, and wavelength variables. The coefficients of the terms in the polynomial are calculated from real and differential ray trace data. The merit function is the variance of the wave error, obtained by squaring and integrating the polynomials over appropriate ranges of the variables. Distortion is removed from the wave error polynomials and is included in the merit function in a unique manner.