Vision, Measurement of

Many people wear glasses or contact lenses. In most cases, this is to correct some inherent defect in vision. The three most common vision defects are farsightedness, nearsightedness, and astigmatism.

Detecting Vision Defects

The most common test for detecting vision defects is the visual acuity test. This is the well-known eye chart with a big letter E at the top. The test is conducted by having the patient stand or sit a certain distance from the chart. The person being tested covers one eye at a time and reads the smallest line of text she or he can see clearly. Each line of text is identified by a pair of numbers such as 20/15, 20/40, and so on. The first number is the distance (in feet) from the eye chart. The second number is the distance at which a person with "normal" vision can read that line. So a visual acuity of 20/40 means that a person with normal vision could read that line from 40 feet away.

Normal vision refers to average vision for the general population and is considered reasonably good vision. Many people have vision that is significantly better than normal. A visual acuity of 20/15 (common in young people) means that the person can read a line at 20 feet that a person with normal vision could only read at 15 feet.

Metric units are used in countries that have adopted the metric system. In those countries, normal visual acuity would be reported as 6/6 (that is, in units of meters where 1 meter equals about 3.28 feet). Minimum levels of visual acuity (for example, required for driving without corrective lenses) would be 6/12.

Astigmatism can be easily detected by having the patient examine a chart such as the one shown on the next page. Astigmatism will cause some of the lines to appear darker or lighter. Some lines may appear to be blurry or double.

Presbyopia is a common condition among older adults. This is the progressive inability to focus on nearby objects. This condition is evident when a person tries to read something by holding it at arm's length. The condition occurs when the lens in the eye becomes less flexible with age.

Corrective Lenses

If a person has visual acuity that is less than adequate, corrective lenses may be appropriate. Eyeglasses and contact lenses are available to correct nearsightedness, farsightedness, and astigmatism.

The human eye is much like a camera. It has a lens and a diaphragm (called the iris) for controlling the amount of light that enters the eye. Instead of film or a Charge Coupled Device (CCD) chip, the eye has a layer of light-sensitive nerve tissue at the back called the retina. The eye does not have a shutter. However, the human nervous system acts much like a shutter by processing individual images received on the retina at the rate of about 30 images per second.

The retina is highly curved (instead of flat, like the film in a camera) but our nervous system is trained from infancy to correctly interpret images from this curved surface. So straight lines are seen as straight lines, vertical lines appear vertical, and the horizon looks like a horizontal line.

The bending of light rays necessary to get the image correctly focused on the curved retina is done mostly by the cornea at the front of the eye. Modern surgical techniques have been developed for altering the shape of the cornea to correct many vision defects without the need for corrective lenses. The lens of the eye provides the small adjustments necessary for focusing at different distances. To focus on distant objects, the ciliary muscles around the lens relax and the lens gets thinner. This lengthens the focal length of the lens so that distant objects are correctly focused. To focus on objects that are near, the ciliary muscles contract, forcing the lens to get thicker and shortening its focal length.

The closest distance at which objects can be sharply focused is called the near point. The far point is the maximum distance at which objects can be seen clearly. An eye is considered normal with a near point of 25 cm (centimeters) and a far point at infinity (able to focus parallel rays). However, these distances change with age. Children can focus on objects as close as 10 cm, while older adults may be able to focus on objects no nearer than 50 cm. Calling any eye "normal" is somewhat misleading, because 67 percent of adults in the United States use glasses or other corrective lenses.

Farsightedness, or hyperopia, refers to a vision defect that causes the eye to be unable to focus on nearby objects because the eye is either too short or the cornea is the wrong shape. For persons with hyperopia, the near point is greater than 25 cm. A similar vision defect is known as pres-byopia. Hyperopia is due to a misshapen eyeball or cornea, whereas presbyopia is due to the gradual stiffening of the crystalline lens. Presbyopia also prevents people from being able to focus on nearby objects, but it affects only the near point. Converging lenses (magnifying lenses) will correct for both hyperopia and presbyopia by making light rays appear to come from objects that are farther away.

Nearsightedness, or myopia, refers to the vision defect that causes an eye to be able to only focus on nearby objects. The far point will be less than infinity. It is caused by an eye that is too long or by a cornea that is incorrectly shaped. In both cases, the images of distant objects fall in front of the retina and appear blurry. Diverging lenses will correct this problem, because they cause parallel rays to diverge as if they came from an object that is closer.

Astigmatism is caused by a lens or cornea that is somewhat distorted so that point objects appear as short lines. The image may be spread out horizontally, vertically, or obliquely. Astigmatism is corrected by using a lens that is also "out of round." The lens is somewhat cylindrical instead of spherical. Frequently, astigmatism occurs along with hyperopia or myopia, so the lens must be ground to compensate for both problems.

Contact lenses work in a way very similar to eyeglasses. However, they have one advantage over eyeglasses. Since the contact lens is much closer to the eye, less correction is needed. This gives the user of contact lenses a wider field of view.

Calculating the power of corrective lenses can be done in some cases using the familiar lens formula. If the focal length of a lens is f, the distance from the lens to the object is d _o, and the distance from the lens to the image is d _i, then:

As an example, suppose a farsighted person has a near point of 100 cm. What power of glasses will this person need to be able to read a newspaper at 25 cm? To solve this we must recognize that the image will be virtual (as in a magnifying glass) so d _i, will have a negative value. The glasses must provide an image that appears to be 100 cm away when the object is 25 cm away.

The power, P, of a lens in diopters (D) is when f is in meters, so .