Page 9 - Practical-Refraction-English

Emmetropia, C Near Vision, Accommodation and Presbyopia Ametropia When an object being viewed is brought closer to the Presbyopia : eye, the image of this object would be formed further An eye is said to be presbyopic (from the Greek meaning old behind the retina unless the power of the eye is increa- eye) when the shape and position of its lens are no longer 1 sed so as to maintain the image on the retina. The eye able to be altered sufficiently to allow sufficient increase in has the capacity to increase its overall power and this is the refractive power of the eye for a clear image of near achieved by changing the surface curvatures, thickness objects to be formed on the retina; that is, when the ampli- and position of the lens; this process is the phenomenon tude of accommodation is insufficient for near visual needs. of accommodation. Left uncorrected, presbyopia will cause near objects to be seen as blurry. The amplitude of accommodation is the range over which the eye can focus. It represents the distance bet- The principle of correction of presbyopia is to supplement ween the furthest object point seen clearly without the insufficiency of the amplitude of accommodation (in accommodation (the Far Point, or Punctum Remotum) near vision) by means of a plus lens. This lens, which is addi- and the closest object point seen clearly with maximum tional to any correction of ametropia, is called a near addi- accommodation (the Near Point, or Punctum Proximum). tion, or more simply an add. Thus: In the emmetropic eye, this accommodation range - the presbyopic emmetropic eye is corrected by a Practical Refraction extends from infinity to the near point (which is a finite plano lens for distance and a plus lens for near; distance). In the myopic eye, the range is real and loca- - the presbyopic myopic eye is corrected by a minus ted entirely at a finite distance in front of the eye. In the lens for distance and a lens which is “less minus” for near hypermetropic eye, the accommodative range is either (this may mean that the near correction may be minus, partly virtual (behind the eye) and partly real (in front of plano or even plus, depending on the level of myopia and the eye) or wholly virtual. the add); - the presbyopic hypermetropic eye is corrected by a The value of the amplitude of accommodation determi- plus lens for distance and a stronger plus lens for near nes the nearest point at which an object may be viewed and for which the eye can form a clear image on its reti- na. The amplitude of accommodation (maximum) is approximately 20 dioptres at birth (corresponding to a near point of ~5cm), >10 dioptres (~10cm) by age 20 years, no more than a few dioptres by age 40 years (~35cm), with a total loss of accommodation by the age of approximately 50 years (depending on various fac- © Essilor International tors). This loss of the ability of the eye to accommoda- te is termed presbyopia. © Essilor International © Essilor International © Essilor International Figure 7: Ametropia and Presbyopia Figure 6: Near Vision 9 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.