Eye Optics and Refractive Errors

Optics and Refractive Errors

Basic Knowledge

Uncorrected and Corrected Visual Acuity

Uncorrected visual acuity: 

This refers to the resolving power of the eyewithout corrective lenses.

Corrected visual acuity: 

This refers to the resolving power of the eye with anoptimal correction provided by corrective lenses (determined by visual acu-ity testing).

Both uncorrected visual acuity and corrected visual acuity provide infor-mation on how far apart two objects must be for the eye to perceive them as distinct objects (minimum threshold resolution). For the eye to perceive two objects as distinct, at least one unstimulated cone must lie between two stimulated cones on the retina. The cone density is greatest in the center of the retina and central visual acuity is highest. There the cones are spaced only 2.5 µm apart. This interval increases toward the periphery of the retina, and both uncorrected visual acuity and corrected visual acuity decrease accordingly. Cone spacing and physical effects such as diffraction and optical aberrations limit the average minimum threshold resolution, the minimumvisual angle to one minute of arc (the individual maximum value is approxi-mately 30 seconds of arc). One minute of arc is 1/60 of a degree or approxi-mately 0.004 mm, which is somewhat more than the width of a cone. This corresponds to the maximum resolving power of the retina (Fig. 16.1).

Refraction: Emmetropia and Ametropia

Refraction is defined as the ratio of the refractive power of the lens and cornea (the refractive media) to the axial length of the globe. Emmetropia is distin-guished from ametropia.

Emmetropia (normal sight): 

The ratio of the axial length of the eye to therefractive power of the cornea and lens is balanced. Parallel light rays that enter the eye therefore meet at a focal point on the retina (Figs. 16.2 and 16.6a) and not anterior or posterior to it, as is the case in ametropia.

Ametropia (refractive error): 

There is a mismatch between the axial lengthof the eye and the refractive power of the lens and cornea. The ametropia is either axial, which is common, or refractive, which is less frequently encoun-tered. The most common disorders are nearsightedness, farsightedness, and astigmatism.

Very few people have refraction of exactly !0.0 diopters. Approximately 55% of persons between the ages of 20 and 30 have refraction between + 1 and –1 diopters.

Emmetropia is not necessarily identical to good visual acuity. The eye may have other disorders that reduce visual acuity, such as atrophy of the optic nerve or amblyopia.

The refractive power of an optical lens system is specified in diopters, which are the international units of measure. Refractive power is calculated accord-ing to the laws of geometric optics. According to Snell’s law, the refraction of the incident light ray is determined by the angle of incidence and difference in the refractive indices n of the two media (Table 16.1).

The maximum total refractive power of an emmetropic eye is 63 diopters with an axial length of the globe measuring 23.5 mm. The cornea accounts for 43 diopters and the lens for 10 – 20 diopters, depending on accommodation. However, the refractive power of the eye is not simply the sum of these two values. The optic media that surround the eye’s lens system and the distance between the lens and cornea render the total system more complex.

The refractive power D (specified in diopters) of an optical system is the reciprocal of the focal length of a lens f (specified in meters). This yields the equation: D = 1/f.

Example: Where a lens focuses parallel incident light rays 0.5 mbehindthelens, the refractive power is 1/0.5 m = + 2 diopters. This is a converging lens. Where the virtual focal point is in front of the lens, the refractive power is 1/–0.5 m = –2 diopters. This is a diverging lens (Fig. 16.3).

Accommodation

The refractive power of the eye described in the previous section is not a con-stant value. The eye’s refractive power must alter to allow visualization of both near and distant objects with sharp contours. This accommodation is made possible by the elasticity of the lens.

Accommodation mechanisms: 

Accommodation involves the lens, zonulefibers, and ciliary muscle.

❖ Lens: The soluble proteins of the lens are surrounded by a thin elastic cap-sule. The curvature of the posterior capsule of the lens is greater than its anterior curvature, with a posterior radius of 6.0 mm as opposed to an anterior radius of 10.0 mm. The intrinsic elasticity of the lens capsule tends to make the lens assume a spherical shape. However, in the unaccommo-dated state this is prevented by the pull of the zonule fibers. The elasticity of the inner tissue of the lens progressively decreases with age due to deposits of insoluble proteins.

❖ Zonule fibers: The radiating zonule fibers insert into the equator of thelens and connect it to the ciliary body. They hold the lens securely in posi-tion and transmit the pull of the ciliary muscle to the lens.

❖Ciliary muscle: Contraction of the ring-shaped ciliary muscledecreases thetension in the zonule fibers. The lens can then approach the spherical shape (with a radius of curvature of 5.3 mm) that its physical configuration and chemical composition would otherwise dictate. This change in the curvature of the lens is especially pronounced in its anterior surface. The deformation increases the refractive power; the focus of the eye shifts to the near field (Fig. 16.4), and close objects take on sharp contours. As the ciliarymuscle relaxes, the tension on the lens increases and the lens flattens. Theresulting decrease in refractive power shifts the focus of the eye into the distance (Fig. 16.4), and distant objects take on sharp contours.

The ciliary muscle is innervated by the short ciliary nerves, postganglionic parasympathetic fibers of the oculomotor nerve. Parasympatholytics such as atropine, scopolamine, and cyclopentolate inhibit the function of the ciliary muscle and therefore prevent accommodation. Referred to as cycloplegics, these medications also cause mydriasis by inhibiting the sphincter pupillae. 

Parasympathomimetics such as pilocarpine cause the ciliary muscle andsphincter pupillae to contract, producing miosis.

When the ciliary muscle is at rest, the zonule fibers are under tension and the eye focuses on distant objects.

Accommodation is regulated by a control loop. The control variable is the sharpness of the retinal image. The system presumably uses the color disper-sion of the retinal image to determine the direction in which accommodation should be corrected.

Range of accommodation: 

This specifies themaximum increase in refractivepower that is possible by accommodation in diopters (Fig. 16.5). In mathemati-cal terms, the range of accommodation is obtained by subtracting near-point refractive power from far-point refractive power. The near point is shortest distance that allows focused vision; the far point describes the farthest point that is still discernible in focus. The near and far points define the range of accommodation; its specific location in space is a function of the refractive power of the eye.

Example: In one patient, the near point lies at 0.1 m and the far point at 1 m.This patient’s range of accommodation is then 10 diopters –1 diopter = 9 diopters.

In an emmetropic eye, the far point is at optical infinity. However, accom-modation can also bring near-field objects into focus (Fig. 16.6b). The elastic-ity of the lens decreases with increasing age, and the range of accommodation decreases accordingly (Fig. 16.5). Presbyopia (physiologic loss of accommo-dation in advancing age) begins when the range of accommodation falls below3 diopters. The gradual loss of accommodation causes the near point torecede; that patient’s arms become “too short for reading”. Depending on age and limitation of accommodation, presbyopia can be compensated for with converging lenses of 0.5 – 3 diopters (see Fig. 16.6c and d).

Adaptation to Differences in Light Intensity

Like a camera, the eye’s aperture and lens system also automatically adapts to differences in light intensity to avoid “overexposure”. This adjustment is effected by two mechanisms.

1. The iris acts as an aperture to control the amount of light entering the eye. This regulation takes about one second and can change the light inten-sity on the retina over a range of about a power of ten.

2. The sensitivity of the retina changes to adapt to differences in light inten-sity. The sensitivity of the retina to light is a function of the concentration ofphotopigment in the photoreceptors and of the neuronal activity of the reti-nal cells. The change in neuronal activity is a rapid process that takes only afew milliseconds and can alter the light sensitivity of the retina over a range of three powers of ten. The change in the concentration of photopig-ment takes several minutes but can cover a wide range of retinal light sen-sitivity, as much as eight powers of ten.