2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 4: Clinical Refraction
Spectacle Correction of Ametropias
Ametropia is a refractive error; it is the absence of emmetropia. The most common method of correcting refractive error is through prescription of spectacle lenses.
Spherical Correcting Lenses and the Far Point Concept
The far point plane of the nonaccommodated eye is conjugate with the retina. For a simple lens (plus or minus sphere), distant objects (those at optical infinity) come into sharp focus at the secondary focal point (F2) of the lens. To correct the refractive error of an eye, a correcting lens must place the image it forms (or its F2) at the eye’s far point. The image at the far point plane becomes the object that is focused onto the retina. For example, in a myopic eye, the far point lies somewhere in front of the eye, between it and optical infinity. In this case, the correct diverging lens forms a virtual image of distant objects at its F2, coincident with the far point of the eye (Fig 4-20).
The same principle holds for the correction of hyperopia (Fig 4-21). However, because the far point plane of a hyperopic eye is behind the retina, a converging lens must be chosen with appropriate power to focus parallel rays of light to the far point plane.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.