If the light rays passing through the two pinholes in Scheiner's Disk come from different sources, then one source can remain fixed while the other is can be repositioned. By adjusting the moveable source horizontally and vertically, the isolated ray of light is redirected until it intersects the fixed ray at the retina and the patient now reports seeing a single point of light.

Having made this adjustment, the displacement distances Dx and Dy are measures of the ray aberration of the eye at the given pupil point. A modern, computerized version of this basic idea which samples the pupil at many locations sequentially was described by Webb, Penny and Thompson in 1992. They called their device a "spatially resolved refractometer".