The first question people usually ask is: why is the technique called the Hartmann-Shack method? The answer is historical precident.

In 1900 Hartmann devised a method for measuring the ray aberrations of mirrors and lenses simply by using a metal disk with holes drilled in it to use as an aperture which would isolate rays of light so they could be traced. Rays that go the wrong way are called aberrated rays, and so the Hartmann screen is a technique for measuring ray aberrations.

Seventy years later Shack and Platt invented a new kind of Hartmann screen made from an array of tiny lenses. Their technique came to be known as Shack's modified Hartmann screen, or Shack-Hartmann for short.

The first use of the Shack-Hartmann method to measure aberrations in human eyes was by Liang and colleagues in 1994. Liang's paper referred to the technique as the Hartmann-Shack method, which has spawned countless arguments about whether the method is properly called the Shack-Hartmann or the Hartmann-Shack. In my view, it is a moot point because the idea actually goes back 300 years prior to Hartmann.

In 1619, nearly 400 years ago, the celebrated Jesuit philosopher Scheiner, who was a contemporary of Kepler and Galileo and who published some 75 years prior to Huygens, described a simple device now known as the Scheiner Disk for demonstrating the focusing mechanism of eyes.