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"If somebody offered me an optical device with all these flaws, I would reject the offer out of hand" Hermann von Helmholtz on the human eye. Heidelberg, 1859 The human eye is a very delicate, complex, and unfortunately imperfect optical system. The most commonly known types of refractive errors of the eye are nearsightedness (myopia), farsightedness (hyperopia), and the cylindrical refractive errors (astigmatism). Similar to every optical system, the human eye also shows higher order refractive errors, the so-called 'higher order aberrations'. The quality of vision is affected more by the higher order aberrations as the light level decreases. For example, our pupils dilate in twilight situations, in order to get more light to the retina. As a result, light rays also pass through the outer regions in the eye, where stronger refractive errors are present. Therefore, even a person with normal 20/20 vision has a decreased visual acuity under critical illumination conditions (see figures). The refractive errors described above may pertain to any healthy eye. In addition, pathological effects can for example lead to opaque corneas or a deformation of the cornea, commonly known as keratoconus. In these cases, vision will not only be significantly degraded but patients might completely loose their eyesight if not treated surgically. |
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