What Is Perfect Vision?

In cases of normal vision, also called "perfect vision" or "20/20 vision," the eye is able to focus light from objects precisely on the retina at the back of the eye. Refractive errors occur when the eye is unable to properly focus light on the retina. In nearsightedness, or myopia, vision is better up close than at a distance. In farsightedness, or hyperopia, vision is better at a distance.

Visual Acuity

• Visual acuity, also known as "Snellen acuity," is clarity of vision at a particular distance. Acuity depends on the sharpness of retinal focus, the sensitivity of visual nerves, and the functionality of the brain's visual areas. In practice, you measure visual acuity by measuring the ability to identify black symbols on a white background at a standardized distance. Visual acuity can vary within a single day. It is not uncommon to have sharper vision in the morning than after several hours of reading.

The Snellen Eye Chart

• The most common chart used to measure visual acuity is the Snellen eye chart, which the Dutch Ophthalmologist Dr. Hermann Snellen devised in 1862. The Snellen chart has lines of letters in different sizes with the largest at the top. Variations on the Snellen chart are used for people who cannot read the alphabet. The Tumbling E chart contains the capital letter "E" in different orientations, and the Broken Wheel test uses cards with pictures of cars with whole or broken wheels.

Snellen Fractions

• Snellen fractions are measures of visual acuity. In a Snellen fraction 20/n, where n is a number, 20 is the distance in feet at which a person can see symbols that an average person can see n feet away. If a person with 20/40 acuity stands 20 feet away from an object, he can see it as well as someone with 20/20 acuity who stands 40 feet away. The smaller n is, the better the acuity, and vice versa. A person is legally blind if her best-corrected vision is 20/200 or worse.

Perfect Vision

• Contrary to a common belief, 20/20 vision is not strictly speaking perfect (see Visual Acuity). Many people have 20/15 or 20/10 vision. 20/20 vision is not necessarily good vision either. People with hyperopia can have 20/20 vision despite being unable to bring nearer objects into focus. Besides sharpness of focus at a distance, good vision requires, among other things, good close-up vision, peripheral vision, depth perception, focusing ability, eye coordination, and color vision. Other tests besides the Snellen test are, therefore, needed to determine quality of vision.

Methods to Improve Vision

• In myopia, the eye is longer than average, causing light to land in front of the retina rather than on the retina. In hyperopia the eye is shorter than average, causing light to land behind the retina. Eye glasses, contact lenses or laser refractive surgery can move the image onto the retina. Some vision experts recommend against corrective devices for myopia. According to David De Angelis, author of The Secret of Perfect Vision, each time you change your prescription, you put stress on your eye muscles, which makes your myopia worse. Exercising the extrinsic ocular muscles, he argues, can reverse and prevent myopia.