When an eye is corrected for distance, how much accommodation is required to focus the image of a near object? Compared with spectacles, contact lenses increase the accommodative demand for myopic eyes and decrease it for hyperopic eyes in proportion to the size of the refractive error.
Clinical Example 5-2 and Figure 5-1 illustrate accommodative demand. For an emmetropic eye to read print that is one-third meter away, it needs to accommodate 3 diopters. Suppose that the eye is myopic, instead of emmetropic, and has a −7.00 D spectacle lens 15 mm in front of it, which exactly corrects its myopia for distance viewing. Parallel rays passing through the lens have divergence of −6.3 D when they reach the cornea, and the eye effortlessly focuses those rays on the retina. A pencil of rays coming from an object one-third meter in front of that eye has about −3 D divergence when it reaches the −7.00 D lens, and therefore −10 D leaving the lens. After traversing the 15 mm from the lens to the cornea, it has −8.7 D divergence. In order to focus the pencil on the retina, which requires the pencil to have −6.3 D divergence at the cornea, the required accommodation is the difference, which is 2.4 D. This is the “near effectivity” of the myopic spectacles, reducing accommodative demand, in this case, from 3 D to 2.4 D. If we correct the same eye with a contact lens, a −6.30 D contact lens is required to replace the −7.00 D spectacle lens for distance viewing, and the full 3 D of accommodation is required to read the print at one-third meter. Thus, contact lenses eliminate the accommodative advantage enjoyed by those with spectacle-corrected myopia and the disadvantage experienced by those with spectacle-corrected hyperopia. A 42-year-old person with myopia who is getting along well enough with −5.00 D glasses may find reading with contact lenses difficult, even though both the glasses and contacts are correct for distance. Conversely, a 38-year-old person with hyperopia, beginning to have symptoms of presbyopia with glasses, will have easier near vision with contact lenses fitted to correct distance vision, than with the glasses.
Depending on their power, spectacle lenses, whose optical centers are positioned for distance viewing, and contact lenses require different amounts of convergence to achieve fusion, looking at a near object. Contact lenses turn with the eyes; therefore, no prism is encountered, looking through the contact lenses, when the eyes converge. Myopic spectacle lenses induce base-in prisms for near objects, following the Prentice rule, so that the eyes do not have to turn in as much to look at the near object. Hyperopic spectacles increase the convergence needed for fusion by inducing base-out prisms.
In summary, correction of myopia with contact lenses, as opposed to spectacle lenses, increases both accommodative and convergence demands of focusing and fusion for near objects, proportional to the size of the refractive error, and decreases both demands in hyperopia (Fig 5-2). These effects may be welcome, unwelcome, or of no consequence, depending on the patient’s muscle balance and ability to accommodate.
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.