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The Image Forming Process

The transparent media of the eye function as a biconvex lens that refracts light entering the eye and focuses images of the external world onto the light sensitive retina.

Refraction

Recall that light rays will bend when passing from one transparent medium into another if the speed of light differs in the two media. However, parallel light rays will pass from air through a transparent body (e.g., flat lens) without bending if the light rays are perpendicular to the lens surface (Figure 14.5, left). If the light strikes the lens surface at an angle, the light rays will be bent in a line perpendicular to the lens surface (Figure 14.5, right).

Figure 14.5 The course of light rays passing through a transparent lens are illustrated. LEFT: The light rays are entering perpendicular to the surface of the lens. RIGHT: The light rays are entering at an angle to the surface of the lens and are being refracted by the lens.

A biconvex lens, which is functionally similar to the eye's lens system, is flat only at its center. The surface of the area surrounding the center is curved and not perpendicular to parallel light rays (Figure 14.6). Consequently, the curved surfaces of a biconvex lens will bend parallel light rays to focus an image of the object emitting the light a short distance behind the lens at its focal point. The image formed is clear only if the curvature of the lens is symmetrical in all meridians and all divergent light rays emitted by a point source converge at the focal point.

Figure 14.6 The light rays emanating from a point source take divergent paths that enter a biconvex lens at different points along the lens surface. The lens refracts the light rays bringing them together at the focal point some distance from the lens.

Figure 14.7 The eye's lens system functions like a biconvex lens and focuses an image on the retina that is inverted, left-right reversed and smaller than the object viewed.

Note that the greater the curvature of the lens surface the greater is its refractive power and the closer is the focused image to the lens. Note also that the image formed is inverted and left-right reversed (Figure 14.7).

The image formed by eye’s lens system is smaller than the object viewed, inverted (upside-down, Figure 14.6), and reversed (right-left, Figure 14.7). As the image is inverted by the lens system, the superior (top) half of each eye’s visual field is projected onto the inferior (bottom) half of each eye’s retina. Also, as the lens produces a reversed image, the temporal half of each visual field is projected onto the nasal half of each eye’s retina¹. Therefore, the temporal (left) hemifield of the left eye is projected onto the nasal (right) half of the left eye’s retina and the nasal (left) hemifield of right eye is projected onto temporal (right) half of the right eye’s retina. Consequently, the left hemifields of both eyes are projected onto the corresponding (right) halves of the two retinas. It is critical that you understand the relationship between the visual field and the retinal areas and realize that corresponding halves of the two monocular visual fields are imaged on corresponding halves of the two retinas. These relationships form the neurological basis for understanding visual field defects.

Lens Accommodation

The eye must be able to change its refractive properties to focus images of both distant and nearby objects on the retina. Distant objects (greater than 30 feet or 9 meters away from the eye) emit or reflect light that can be focused on the retina in a normal relaxed eye (Figure 14.8).

 

 

 

Figure 14.8 The normal eye at rest can focus on the retina images of objects more than 30 ft from the eye. When an object is brought closer to the eye (i.e., less than 30 ft from the eye), the light rays from the object take more divergent paths and each enters the cornea with a greater angle of incidence. Consequently, the image focal point would be beyond the retina if the eye's lens system were not adjusted. During accommodation, the lens curvature increases, increasing the refractive power of the eye and focusing the image on the retina.

If a viewed object is brought closer to the eye, the light rays from the object diverge at a greater angle relative to the eye (Figure 14.8). Consequently, the nearer the object of view, the greater the angle of incidence of light rays on the cornea, and the greater the refractive power required to focus the light rays on the retina. The cornea has a fixed refractive power (i.e. it cannot change its shape). However, altering the tension of the zonules on the elastic lens capsule can alter the lens shape. The change in the refractive properties of the eye is called the accommodation or "near point" process.

In the normal eye under resting (distant vision) conditions, the ciliary muscles are relaxed and the zonules are under tension (Figure 14.9). In this case, the lens is flattened, which reduces the refractive power of the lens to focus on distant objects. When an object is closer to the eye (i.e., less than 30 ft. away), accommodation occurs to affect “near vision”. The ciliary muscle contracts, pulling the ciliary processes toward the lens (remember the muscle acts as a sphincter). This action releases tension on the zonules and the lens capsule. The reduced tension allows the lens to become more spherical (i.e., increase its curvature). The increase in lens curvature increases the lens refractive power to focus on near objects. Consequently, as an object is moved closer to the viewer, his eyes accommodate to increase the lens curvature, which increases the refractive power of his eye (Figure 14.8).

Figure 14.9 During distance vision (i.e., with the eye at rest), the ciliary muscles are relaxed and the zonules are under tension. The lens is flattened by the tension on the zonules and the lens capsule. However, in the accommodation process, the ciliary muscles contract and, acting like a sphincter muscle, decrease the tension on the zonules and lens capsule. The lens becomes more spherical with its anterior surface shifting more anteriorly into the anterior chamber.

Refractive Errors of the Eye and Corrective Lenses

Presbyopia: In presbyopia, there is normal distance vision, but lens accommodation is reduced with age. With age, the lens loses its elasticity and becomes a relatively solid mass. During accommodation, the lens is unable to assume a more spherical shape and is unable to increase its refractive power for near vision (Figure 14.10). As a result, when an object is less than 30 ft. away from the presbyopic viewer, the image is focused somewhere behind the retina.

Figure 14.10 In the presbyopic eye, when the object is moved closer to the eye, the lens is unable to accommodate and the image is focused beyond the retina. For the presbyopic eye a corrective lens that converges the light rays (i.e., a convex lens that reduces the angle of incidence of light on the cornea) will allow the presbyopic eye to view nearby objects.

A convex lens (i.e., increased refractive power) is used to correct the presbyopic eye (Figure 14.10). These lenses refract the light rays so they strike the surface of the cornea at a smaller angle. However, because the corrective lens increases the refractive power, the presbyope with convex lenses will have problems with distance vision. Consequently, the corrective lenses are often half lenses (i.e., reading glasses) which allow the presbyope to view objects in the distance unimpeded by the convex lens.

Hyperopia: In hyperopia (Figure 14.11), the refractive power of the eye’s lens system is too weak or the eyeball too short. When viewing distant objects, the image is focused at a point beyond the retina.

Figure 14.11 The hyperopic eye at rest cannot focus on the retina the image of an object more than 30 ft from the eye. The hyperopic lens system is too weak and the image is focused beyond the retina.

The young hyperope can compensate by using lens accommodation, i.e., increase the refractive power of the eye’s lens system (Figure 14.12). We call the hyperope "far-sighted" (hypermetropic) because the power of accommodation used for distance vision cannot be used for near vision.

Figure 14.12 If the hyperopia is not severe; the hyperopic eye can use the lens accommodation process to increase the refractive power of the eye for distance vision.

As the hyperope ages and becomes presbyopic, the power of accommodation is diminished. Consequently, the middle aged hyperope may have a limited range (near and far) of vision. To correct this effect of aging, the refractive power of the eye is increased with convex lenses (Figure 14.12).

Myopia: In myopia (Figure 14.13), the refractive power of the eye’s lens system is too strong or the eyeball too long. When viewing distant objects, the image is focused at a point in front of retina.

Figure 14.13 The myopic eye at rest cannot focus on the retina the image of an object more than 30 ft. from the eye. The refractive power of the eye's lens system is too strong and the image is focused in front of the retina.

The uncorrected myopic eye is "near-sighted" because it can focus unaided on near objects. That is, the young myope will see distant objects as blurred, poorly defined images but can see nearby small objects clearly (remember nearby objects emit divergent light rays).

For distance vision, the refractive power of the myopic eye lens system is corrected with concave lenses that diverge the light rays entering the eye (Figure 14.14). Note that as the power of accommodation diminishes with age, near vision is also affected in the presbyopic-myopic eye. The mature myope may require bifocals, the upper half of the lens diverging light rays for distance vision and the lower half with no or low converging power for near vision.

Figure 14.14 A corrective lens that diverges light rays before they enter the eye (i.e., a concave lens) will allow the myopic eye to focus the image of a distant object on the retina.

Astigmatism: An astigmatism results when the cornea surface does not resemble the surface of a sphere (e.g. is more oblong). In an eye with astigmatism, the image of distant and near objects cannot be focused on the retina (Figure 14.15). Astigmatism is corrected with a cylindrical lens having a curvature that corrects for the corneal astigmatism. The cylindrical lens directs light waves through the astigmatic cornea to focus a single, clear image on the retina.

Figure 14.15 The astigmatic lens is asymmetrical and has multiple focal points, which produces multiple images of a point source.