14 - Ophthalmic lenses

Summary

Introduction

The primary role of an ophthalmic lens is to correct a refractive error of the eye, thus allowing the eye to clearly see objects at a chosen distance. The refractive error may be due to myopia, hyperopia, presbyopia or astigmatic errors and these have been explained in Chapter 13. However, optical refractive corrections have some side effects such as altering the effective positions of the near and far points, altering retinal image sizes, making it more difficult to satisfactorily use visual instruments and finally their aberrations may lead to reduced visual performance. These different aspects of ophthalmic lenses will now be discussed in detail.

Spectacle lenses, contact lenses or intra-ocular lenses

Either spectacle or contact lenses may be used to correct refractive errors. Both have their advantages and disadvantages. For example, spectacle lenses have little or no biological interaction with the tissues of the eye and therefore cause less or no biological reaction. However, they have a more restricted visual field and affect the size of the retinal image. This change in retinal image size is called spectacle magnification and the magnitude increases with lens power and distance of the lens from the eye or more strictly, from the entrance pupil. Contact lenses also have spectacle magnification but since these are much closer to the pupil, their spectacle magnification is much less than that of spectacle lenses. Intra-ocular lenses are artificial lenses inserted in the eye to replace the original lens after it has been removed, usually because of a cataract. Because these lenses are placed close to or in the same position as the original lens, that is close to the pupil, their spectacle magnification is almost zero.