A subjective method for the measurement of monochromatic* aberrations of the eye

TLDR

The study introduces an aberroscope that employs artificial astigmatism instead of myopia to defocus a point source, enabling measurement of monochromatic aberrations. Subjects view a point source through a ±5 D crossed‑cylinder lens that casts a distorted grid shadow; the distortions are quantified by fitting a fourth‑order two‑dimensional Taylor polynomial to reconstruct the eye’s wave‑aberration surface. Analysis of over 50 subjects shows that third‑order terms dominate monochromatic aberrations, spherical aberration often appears cylindrical, and aberrations degrade the optical MTF more than previously thought.

Abstract

We have designed an aberroscope that differs from Tscherning's classical instrument in that it makes use of an artificial astigmatism rather than an artificial myopia to defocus the image of a point source of light. A subject views the source through a ±5 D crossed cylinder lens with axes at 45° to the principal axes of an intercalated grid and sees a shadow image of the grid. The distortions of this grid image are quantitatively related to the wave aberration of the eye. Using this device we have obtained drawings for more than 50 subjects. These drawings of the grid pattern have been analyzed by means of a two-dimensional polynomial curve Fitting technique that computes Taylor polynomial terms to the fourth order. From the Taylor coefficients it is possible to reconstruct the wave aberration surface. Examination of the Taylor terms so obtained shows that the monochromatic aberrations of the eye are dominated by third-order Taylor terms within the range of physiological pupil sizes, and that spherical aberration frequently appears predominantly about one axis only, a condition that we have termed "cylindrical" aberration. We have computed the optical MTF of our subjects' eyes and find that the role of aberrations in degrading the MTF may be greater than generally believed.