Eyeglass Prescription - Background

Background

Blur is the subjective experience or perception of a defocus aberration within the eye. Blur may appear differently depending on the amount and type of refractive error. The following are some examples of blurred images that may result from refractive errors:

Blur is corrected by focusing light on the retina. This may be done with eyeglasses or contact lenses, or by altering the shape of various eye structures via refractive surgery or special contact lenses.

Eyeglasses sometimes have unwanted effects including magnification or reduction, distortion, color fringes, altered depth perception, etc. Although many people think of lenses as magnifiers, the lenses within eyeglasses improve vision primarily by reducing blur. Depending on the optical setup, they may also produce magnification or reduction of images which may or may not be intentional or desirable. Often, magnifiers are part of a regimen prescribed by low vision optometrists to help people with reduced vision.

The visual acuity is measured with an eye chart. The eye chart is the background used by eye doctors to compare the patient's visual acuity with the one of other human beings. Although there are many variations of the eye chart, the standard one is the Snellen eye chart, which was developed by Dutch eye doctor Hermann Snellen in the 1860s. Usually, these charts show 11 rows of capital letters and it is common that the first row contains one letter (the "big E") and the other rows contain letters that are progressively smaller. Other types of chart eyes are the Landolt C and the Lea test.

Individuals who are not able to read letters for various reasons including being too young to know the alphabet or having a handicap, eye doctors may use what is called the tumbling E chart. This type of chart is a variation of the Snellen chart and it shows the capital letter E, at different sizes and rotated in increments of 90 degrees. The scale of the tumbling E chart is the same as with the standard Snellen chart. The eye doctor, in this case, will ask the person being tested to use either hand (with their fingers extended) to show which direction the "fingers" of the E are pointing: right, left, up or down.

In the United States, a 20/20 visual acuity is considered normal. This means that the chart is normally placed at 20 feet distance from the person who is being tested. 20/20 visual acuity is considered normal vision for individuals but not perfect as some individuals, although rarely, can see at 20 feet what others can see at 10. The poorest visual acuity is 20/200 and a person with the best-corrected vision, or vision once wearing corrected lenses, of 20/200 is normally considered legally blind. Individuals with 20/200 vision are normally able to read only the first letter on the chart. Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better. In the United States individuals who want to get their driver license without a corrective lenses restriction must have at least 20/40 visual acuity.

Eye charts do not provide information on the peripheral vision, depth perception or color perception and therefore they are not sufficient characterize the quality of vision, nor to assess the health of the eyes. A complete eye examination will include other tests. However, eye charts are useful in deciding whether the patients need eyeglasses or contact lenses to correct their distance vision, and assessing how effective their refractive correction is.