The Normal (Emmetropic) Eye

An eyeglass lens supplements the eye. It helps the eye perform functions that the eye cannot do itself. Therefore, to understand the function of lenses, you must understand how the normal eye works. Figure 3-1 shows the parts of the normal, or emmetropic
(em i TROP ic), eye. Study it carefully before you proceed.

Figure 3-1 The Normal Emmetropic Eye

Figure 3-1 also shows light as it enters the eye from distant objects. Rays of light from distant objects proceed to the eye along parallel lines. When they reach the eye they are bent, or refracted, by the cornea. These rays of light are refracted even further by the crystalline lens, and are sharply focused on the retina at the back of the eye.

The Farsighted Eye

There are a variety of reasons why light will not focus precisely on the retina. Some of these reasons have to do with the shape of the eyeball itself. For example, people who have little problem with distance vision, but do have problems close-up are called far-sighted, or hyperopes (HY pe rop). The hyperopic (hy per O pik) eye may be thought of as an optically weak power system. The result is that light comes to a focus behind the retina, causing an image to blur. This problem is illustrated in Figure 3-2.

Figure 3-2 The Farsighted Eye

Figure 3-3 shows an eye which has been corrected for hyperopia (hi pe RO pe a) by adding a converging lens. The lens adds convergence to the optical system of the eye by redirecting the focus of the light forward to the retina to produce a clear image. Converging lenses are also called convex, or plus, lenses. With the normal accommodation of the crystalline lens, and the help of the corrective lens, the eye can now focus clearly upon near and distant objects with little eye strain.

Figure 3-3 The Farsighted Eye - Corrected

The Nearsighted Eye

The other common condition which causes blurred vision is near-sightedness, or myopia (mi O pe a). The nearsighted eye may be considered to be optically too strong to allow light to focus on the retina. The nearsighted eye is shown in Figure 3-4.

Figure 3-4 The Nearsighted Eye

Although a nearsighted person may be able to see clearly objects held close to the eye, objects in the distance will appear blurred. Nearsightedness is corrected using a diverging lens (concave or minus) as shown in Figure 3-5. The diverging lens spreads the rays of light farther apart so that they converge on the retina rather than in front of it.

Figure 3-5 The Nearsighted Eye Corrected

Astigmatism

Up to this point, the lenses in our sample eyeballs have been spherical in nature. Spherical lenses have the same curvature in all directions. However, it is not uncommon for the cornea of the eye to have slightly varying curvature. This produces a condition called astigmatism (a STIG ma tism) which cannot be corrected with a simple spherical eyeglass lens. The uncorrected eye of a patient with astigmatism will not bring light rays to a sharp, well-defined focus on the retina, Figure 3-6. Objects may appear out of focus differently at right angles.

To correct for astigmatism, a cylindrical surface is ground with a spherical surface to produce a lens with maximum and minimum refractive powers 90 degrees from each other. This sphero-cylinder lens, when properly oriented before the eye, will cancel the eye’s astigmatic refractive power error.

Figure 3-6 Astigmatism

Presbyopia

The two lens types considered so far produce a correction primarily for distant objects. Such lenses are called single-vision lenses. They rely upon the eye’s own crystalline lens to produce clear images of near objects.

The crystalline lens may be stretched or relaxed by the eye muscles. This process is called accommodation. Accommodation allows the normal eye to produce clear images on the retina of objects that are relatively near to the eye as well as those that are distant. The process works in much the same way as the focusing action of a camera.

As people approach middle age, their crystalline lenses lose some elasticity and they begin to have difficulty focusing on books, newspapers, etc. This condition is called Presbyopia (prez be O pe a). To solve this condition, lenses with more than one viewing range are used. These lenses are called bifocals, trifocals, progressives, or, more generally, multifocals (Figure 3-7). A bifocal lens is really two lenses combined into one. The lens provides a distance-viewing area and a near-viewing area. A trifocal provides three viewing areas: distance, intermediate and near. A progressive lens provides a continuous increase in lens power as the eye moves down from the distance viewing to the near viewing range. Progressive lenses are sometimes called "no-line" bifocals. You will learn more about lenses in another unit of this course.

Figure 3-7 Bifocal, Trifocal and Progressive Lenses

Review of Simple Lenses

Figure 3-8 is an illustration of a plus lens, also called a converging lens, because in hyperopes it causes light to converge to a point on the retina rather than behind it.


Figure 3-8 The Plus (Converging) Lens

The Spherical (Minus)

The minus lens causes light to diverge. Remember, the minus lens is used for people who are nearsighted. The light rays are pushed apart (diverged) so that they focus upon the retina rather than in front. Figure 3-9 shows a spherical minus lens. See how parallel light rays are pushed apart.

Figure 3-9 The Minus (Diverging) Lens

In order to make ophthalmic lenses more optically correct, and more attractive, their outside surfaces are made convex and their inside surfaces are made concave. See Figure 3-10, Modern Lens Form.

Figure 3-10 Modern Lens Form

Sphero-cylinder Lens

The lens to correct astigmatism is sometimes called a sphero-cylinder. The front surface is spherical while the back surface is cylindrical. This combination of front and back surfaces creates a lens prescription of spherical power with a cylinder amount ground on the back surface to correct the two focus points created by the astigmatism. The cylinder amount on the back surface is ground in the required position, called the axis. Figure 3-11 shows a sphero-cylinder lens.

Figure 3-11 Sphero-cylinder Lens Astigmatism Corrected

Index of Refraction

The subject of index of refraction is complex and will only be introduced here. It will be covered in more detail in the course which deals with surface room operations. For now, simply remember that the four quantities which control the refractive power of an ophthalmic lens are:

• the curvature of the front surface (closest to the object being viewed);

• the curvature of the rear surface (closest to the eye);

• index of refraction of the material from which the lens is made; and

• the thickness of the lens.