ABSTRACT
Ametropia and presbyopia are experienced by up to one-third of the world’s population. Objective presbyopia (loss of accommodation with binocular near vision <N8 and improving to ≥N8 with near addition lenses) alone affects almost all people over the age of 45. Thus, everyone over this age will need correction for near vision, distance vision, or both. If uncorrected, refractive errors can be responsible for anything from mild visual impairment to blindness. It has also been shown that poor visual acuity affects quality of life regardless of the cause (Brown et al. 2002). Consequently, reduced visual acuity as a result of uncorrected
Contents 19.1 Introduction 277 19.2 Significance of refractive error 277
19.2.1 Prevalence of ametropia 278 19.2.2 Prevalence of presbyopia 278 19.2.3 Prevalence of uncorrected refractive error 278
19.3 Emmetropia 279 19.4 Ametropia 279
19.4.1 Myopia 279 19.4.1.1 Effect of a pinhole 279
19.4.2 Hypermetropia 280 19.4.2.1 Effect of a pinhole 281
19.4.3 Astigmatism 281 19.4.3.1 Sturm’s conoid 281 19.4.3.2 Types of regular astigmatism 282
19.4.4 Axial versus refractive ametropia 285 19.5 Correction of ametropia 286
19.5.1 Relative spectacle magnification 287 19.5.2 Anisometropia 288
19.6 Presbyopia 289 19.7 Correction of presbyopia 289 19.8 Concluding comments 290 References 290
refractive error has the same effect on quality of life as reduced acuity resulting from ocular pathology despite the ease of its correction. All refractive errors are easily corrected with spectacles, contact lenses, or refractive surgery in most cases; however, in the small proportion with high myopia, there is the potential for other, irreversible, blinding conditions such as retinal detachments and myopic macular degeneration (Foster and Jiang 2014, Wong et al. 2014). Since health has been defined as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity” (WHO 1946), refractive errors represent a major health concern.
