Figure 4-2 Accommodative demand. A, Effective spectacle lens power at the corneal surface. B, Accommodative demand with a –7.0 D spectacle lens. C, Correction with a –6.3 D contact lens. D,
Accommodative demand with a –6.3 D contact lens. (Illustrations developed b y Thomas F. Mauger, MD.)
The vergence of rays originating at a distance of 33.3 cm after exiting the spectacle lens is –10 D (Fig 4-2B). The vergence is calculated by using the vergence of the light after it leaves the spectacle lens: –3 + (–7) = –10. Due to the vertex distance, the vergence of these rays at the front surface of the cornea (which is approximately the location of the first principal point) is –8.7 D. Use the focal point of the vergence after the light travels through the lens, –10 D, 1/10 = 0.1 m, plus the vertex distance of 0.015 m (0.115 m) to find the vergence at the corneal surface: 1/0.115 m = –8.7 D).
Accommodation is the difference between the vergence at the first principal point between rays originating at infinity and the vergence of rays originating at a distance of 33.3 cm. In this case, the accommodation is 2.4 D: –6.3 – (–8.7) = 2.4. In contrast, the accommodation required with a contact lens correction is approximately 3 D (Fig 4-2C, D). Therefore, this myopic eye would need 0.6 D more accommodation to focus an object at 33.3 cm when wearing a contact lens compared with correction with a spectacle lens. Similarly, the accommodative demands of an eye corrected with a +7 D spectacle lens would be 3.5 D compared with approximately 3 D for a contact lens (Table 4-2).
Table 4-2
Convergence Demands
Depending on their power, spectacle lenses (optically centered for distance) and contact lenses require different convergences. Myopic spectacle lenses induce base-in prisms for near objects. This benefit is eliminated with contact lenses. Conversely, hyperopic spectacles increase the convergence demands by inducing base-out prisms. In this case, contact lenses provide a benefit by eliminating the incremental convergence requirement.
In summary, correction of myopia with contact lenses, as opposed to spectacle lenses, increases both accommodative and convergence demands of focusing near objects proportional to the size of
the refractive error. The reverse is true in hyperopia (Fig 4-3).
Figure 4-3 Effect of spectacle lenses on convergence demands. A, Lenses for correction of hyperopia create induced base-out prism with convergence, which increases the convergence demand. B, Lenses for correction of myopia create induced base-in prism, which decreases the convergence demand. (Illustrations developed b y Thomas F. Mauger, MD.)
Tear Lens
The presence of fluid, rather than air, between a contact lens and the corneal surface is responsible for another major difference between the optical performance of contact lenses and that of spectacle lenses. The tear layer between a contact lens and the corneal surface is an optical lens in its own right. As with all lenses, the power of this tear, or fluid, lens is determined by the curvatures of the anterior surface (formed by the back surface of the contact lens) and the posterior surface (formed by the front surface of the cornea). Because flexible (soft) contact lenses conform to the shape of the cornea and the curvatures of the anterior and posterior surfaces of the intervening tear layer are identical, the power of their tear lenses is always plano. This statement is not generally true of rigid contact lenses: the shape of the posterior surface (which defines the anterior surface of the tear lens) can differ from the shape of the underlying cornea (which forms the posterior surface of the tear lens). Under these circumstances, the tear layer introduces power that is added to the eye’s optical system.
The power of the tear lens is approximately 0.25 D for every 0.05-mm radius-of-curvature difference between the base curve of the contact lens and the central curvature of the cornea (K), and this power becomes somewhat greater for corneas steeper than 7.00 mm. Tear lenses created by rigid contact lenses with base curves that are steeper than K (ie, have a smaller radius of curvature) have plus power, whereas tear lenses formed by base curves that are flatter than K (ie, have a larger radius of curvature) have minus power (Fig 4-4). Therefore, the power of a rigid contact lens must account for both the eye’s refractive error and the power introduced by the tear lens. An easy way of remembering this is to use the rules steeper add minus (SAM) and flatter add plus (FAP) (Clinical Example 4-3).
Figure 4-4 A rigid contact lens creates a tear (or fluid) lens whose power is determined by the difference between the curvature of the cornea (K) and that of the base curve of the contact lens. (Courtesy of Perry Rosenthal, MD. Redrawn b y Christine
Gralapp.)
Clinical Example 4-3
The refractive error of an eye is –3.00 D, the K measurement is 7.80 mm (43.25 D), and
the base curve chosen for the rigid contact lens is 7.95 mm (42.50 D). What is the anticipated power of the contact lens?
The power of the resulting tear lens is –0.75 D. This power would correct –0.75 D of the refractive error. Therefore, the remaining refractive error that the contact lens is required to correct is –2.25 D (recall the FAP rule: flatter add plus). Conversely, if the refractive error were +3.00 D (hyperopia), then the necessary contact lens power would be +3.75 D to correct the refractive error and the –0.75 D tear lens (Fig 4-5).
Figure 4-5 Determining the power of a contact lens using the FAP-SAM rules. (Illustration developed b y Thomas F.
Mauger, MD.)
Because the refractive index of the tear lens (1.336) is almost identical to that of a cornea (1.3765), the anterior surface of the tear lens virtually masks the optical effect of the corneal surface. If the back surface of a contact lens is spherical, then the anterior surface of the tear lens is also spherical, regardless of the corneal topography. In other words, the tear layer created by a spherical rigid contact lens neutralizes more than 90% of regular and irregular corneal astigmatism. This principle simplifies the calculation of the tear lens power on astigmatic corneas: because the powers of the steeper corneal meridians are effectively neutralized, they can be ignored, and only the flattest meridians need to be considered. The refractive error along the flattest meridian is represented by the spherical component of refractive errors expressed in minus cylinder form. For this reason, clinicians should use only the minus cylinder format when dealing with contact lenses (Clinical Example 4-4).
Clinical Example 4-4
The refractive correction is –3.50 +1.75 × 90, and the K measurements along the 2 principal meridians are 7.80 mm horizontal (43.25 D at 180°) and 7.50 mm vertical (45.00 D at 90°). The contact lens base curve is 7.50 mm. What is the anticipated power of the contact lens?
The refractive correction along the flattest corneal meridian (7.80 mm) is –1.75 D (convert the refractive error to minus cylinder form), and the lens has been fitted steeper than flat K, creating a tear lens of +1.75 D. Thus, a corresponding amount of minus power must be added (recall the SAM rule: steeper add minus), giving a corrective power of –3.50 D in that meridian.
The refractive correction along the steepest meridian (7.50 mm) is –3.50 D. The lens is fitted “on K”; therefore, no tear lens power is created. The corrective power for this meridian is also –3.50 D.
Accordingly, the power of the contact lens should be –3.50 D (Fig 4-6).