The lens of the eye refracts light waves to bring the image into focus on the retina. The ciliary muscle contracts to make the lens more spherical, increasing its convexity and strengthening the refraction.

To properly image light on the retina, the iris adjusts the amount of light entering the eye through the pupil. Similar to the aperture in a camera, the pupil controls the depth of field and the spherical aberration produced by the lens. Constriction of the pupil increases the depth of field and directs light through the central part of the lens where spherical aberration is minimal. Pupillary constriction occurs reflexively during near vision or in response to bright light.

The retina is composed of ten layers. The pigmented epithelium is the outermost layer, located just inside the choroid. Pigment cells in this layer have tentacle-like processes that extend into the photoreceptor layer, surrounding the outer segments of the rods and cones. These processes prevent transverse scatter of light between photoreceptors and maintain contact between layers 1 and 2. The pigmented epithelium provides nutrients, removes waste from the photoreceptors, phagocytoses shed ends of outer segments of the rods, and converts metabolized visual pigment into a reusable form after transporting it back to the photoreceptors.

As individuals age, the lens loses elasticity, resulting in a decline in accommodation for near vision known as presbyopia. While young people can change the power of the lens by up to 14D, by the age of 40, the amount of accommodation is halved, and by 50 years, it can decrease to 2D or less. Presbyopia can be corrected with convex lenses.

Defects in focus can be caused by a discrepancy between the size of the eye and the refractive power of the dioptric media. Myopia occurs when distant objects are focused in front of the retina and can be corrected with concave lenses. Hypermetropia, on the other hand, causes distant objects to be focused behind the retina and can be corrected with convex lenses. Astigmatism, characterized by an asymmetry in the curvature of different meridians of the cornea or lens (sometimes the retina), can often be corrected with lenses possessing complementary curvature radii.
CHAPTER 8 The Special Senses occurs; that is, until the ciliary muscle contracts, causing the lens to become more spherical, the increased convexity causes the lens to refract the light waves more strongly, bringing the image into focus on the retina.
Proper imaging of light on the retina depends not only on the lens and cornea but also on the iris, which adjusts the amount of light that can enter the eye through the pupil. In this regard, the pupil is analogous to the aperture in a camera, which also controls the depth of field of the image and the amount of spherical aberration produced by the lens. When the pupil is constricted, the depth of field is increased, and the light is directed through the central part of the lens, where spherical aberration is minimal. Pupillary constriction occurs reflexively when the eye accommodates for near vision or adapts to bright light, or both. Thus when a person reads or does other fine visual work, the quality of the image is improved by adequate light.
Layers of the Retina
The 10 layers of the retina are shown in
Fig. 8.2 . The outermost portion is the pigmented epithelium (layer 1), which is just inside the choroid. The pigment cells have
Asanindividualages,theelasticityofthelensgraduallydeclines.Asaresult,accommodationofthelensfornearvisionbecomesprogressivelylesseffective,aconditioncalledpresbyopia. Ayoungpersoncanchangethepowerofthelensbyasmuchas14D.However,bythetimethatapersonreaches40yearsofage,theamountofaccommodationhalves,andafter50years,itcandecreaseto2Dorless.Presbyopiacanbecorrectedbyconvexlenses.
Defectsinfocuscanalsobecausedbyadiscrepancybetweenthesizeoftheeyeandtherefractivepowerofthedioptricmedia.Forexample,inmyopia (near-sightedness),theimagesofdistantobjectsarefocusedinfrontoftheretina.Concavelensescorrectthisproblem.Conversely,inhypermetropia (far-sightedness),theimagesofdistantobjectsarefocusedbehindtheretina;thisproblemcanbecorrectedwithconvexlenses.Inastigmatism, anasymmetryexistsintheradiiofcurvatureofdifferentmeridiansofthecorneaorlens(orsometimesoftheretina).Astigmatismcanoftenbecorrectedwithlensesthatpossesscomplementaryradiiofcurvature.
•Fig. 8.2 Layers of the Retina. Lightimpingingontheretinawouldbecomingfromthetopofthefigure,andwouldpassthroughallthesuperficiallayerstoreachthephotoreceptorrodsandcones.9 Nerve fiber layer Axons at surface of retina passing via optic nerve, chiasm and tract to lateral geniculate body Ganglion cell Bipolar cell Amacrine cell Horizontal cell Rod Cone Choroid Pigment cells Müller cell (supporting glial cell) 8 Ganglion cell layer 7 Inner plexiform layer 5 Outer plexiform layer 6 Inner nuclear layer 4 Outer nuclear layer 2 Photoreceptor layer 1 Pigment epithelium 3 Outer limiting membrane 10 Inner limiting membrane Inner segment Outer segment tentacle-like processes that extend into the photoreceptor layer (layer 2) and surround the outer segments of the rods and cones. These processes prevent transverse scatter of light between photoreceptors. In addition, they serve a mechanical function in maintaining contact between layers 1 and 2 so that the pigmented epithelium can (1) provide nutrients and remove waste from the photoreceptors; (2) phagocytose the ends of the outer segments of the rods, which are continuously shed; and (3) reconvert metabolized visual pigment into a form that can be reused after it is transported back to the photoreceptors.