Abstract

The eye can be treated as an optical instrument. Indeed, it is often compared to a camera or, perhaps more accurately, a video ­camera. The lack of homogeneity of its media and the ­misalignment of its elements, however, set it apart from other instruments. Helmholtz once said of the eye (quoted in Fishman) that “it is not too much to say that if an optician wanted to sell me an instrument that had all these defects, I should think myself quite justified in blaming his carelessness in the strongest terms, and giving him back his instrument” (Fishman 2010). However, despite the lack of homogeneity of its media and the misalignment of its elements, which differentiates the eye from other optical instruments and which “offended” Helmholtz’s sense of optical order, it is capable of creating a sharp image on the retina. Under normal conditions, the eye can provide more than adequate acuity and can adjust for various viewing distances. In practice, if not theory, it is a very effective optical instrument. Nevertheless there are several factors that either individually or in combination can contribute to a blurred image, with a resulting reduction in visual acuity. Unlike the theoretical errors referred to by Helmholtz, these are variations from the normal functioning of the eye that do affect vision. While the optics of the normal eye are fully covered elsewhere in this book, this chapter looks at the various forms of refractive errors that can affect vision and their modes of correction.