Image quality of the human eye.

The eye is an optical instrument that projects scenes of the visual world onto the retina. It has been known for many years that the eye is far from being a perfect optical system, in particular for large pupil diameters. Refractive anomalies (defocus or astigmatism) occur frequently in the eye. In Western countries, myopia affects approximately 30% of the population, although its prevalence is much higher (> 80%) in certain Asian societies. However, the eye suffers also from other optical imperfections (called high-order aberrations), which are not typically measured in the clinic and cannot be corrected by conventional means. Like defocus, optical aberrations blur the retinal image, reducing image contrast and limiting the range of spatial frequencies available to further stages of the visual processing. The contribution of aberrations to optical degradation is typically smaller than is that of defocus or astigmatism. The blurring effect of aberrations becomes more noticeable for large pupils. For small pupil sizes, diffraction effects, associated with limited aperture size, predominate over the aberrations. Along with diffraction and aberrations, scattering also contributes to degradation of retinal image quality. Scattering occurs at the cornea and particularly in the lens. Although typically scattering is small in normal, young eyes, it is well known that it increases with age (due to changes in the crystalline lens) and after PRK refractive surgery.