The shape of the aberrated wavefront is a fundamental description of the optical quality of the eye called the "wavefront aberration function". This function lies at the heart of a rich optical theory that allows us to calculate the retinal image of any object, to assess the quality of that retinal image quantitatively, and ultimately to predict visual performance on visual tasks.

However, to apply this wonderful optical theory we need to analyze the wavefront as soon as it passes through the eye's pupil. To do this we use a pair of relay lenses which focus the lenslet array onto the pupil of the eye.

Optically, then, the lenslet array appears to reside inside the eye, right in the pupil plane where it can subdivide the reflected wavefront immediately as it emerges from the eye's pupil.

This final configuration is the basic form of the modern Hartmann-Shack aberrometer described by Liang in 1994.