Eyeglasses & Contact Lenses

Eyeglasses

Eyeglasses are a frame that holds two pieces of glass or plastic, which have been ground into lenses to correct refractive errors. Refractive errors can include nearsightedness or myopia (difficulty seeing far away), farsightedness or hyperopia (difficulty seeing close up), and astigmatism (blurring due to an irregularly shaped cornea). Eyeglasses perform this function by adding or subtracting focusing power to the eye's cornea and lens.

An Eyeglass Prescription

The lens power of eyeglasses is measured in diopters. This measurement reflects the amount of power necessary to focus images directly on to the retina. When looking at an eyeglasses prescription, you will see the following abbreviations:

• O.D. - Oculus dextrus refers to the right eye (sometimes the abbreviation RE is used)
• O.S. - Oculus sinister refers to the left eye (sometimes the abbreviation LE is used)

In addition, the eyeglass prescription may contain the following measurements:

• Sphere - the extent of the nearsightedness or farsightedness
• Cylinder - the amount of astigmatism in the eye
• Axis – description of the astigmatism in degrees from the horizontal axis (most left and right eyes have the same axis in astigmatism)

Bifocal prescriptions, which correct both nearsightedness and farsightedness, usually have an additional measurement listed on the prescription as "add," to indicate the strength of the lens. By federal law, an eye doctor is required to provide an eyeglass prescription following an eye exam at no extra cost. In addition, under federal law, you have the right to a copy of your eyeglass prescription, so you can shop for the best value in eyewear.

Types of Eyeglass Lenses

The type of lenses used in eyeglasses depends on the type of vision problem and may include:

• Concave lenses (thinnest in the center) to correct nearsightedness (myopia)
• The numerical prescription in diopters is always marked with a minus (-) symbol
• Convex lenses (thickest in the center) to correct farsightedness (hyperopia)
• The numerical prescription in diopters is always marked with a plus (+) symbol
• Cylindrical lenses curve more in one direction than the other and are often used to correct astigmatism

Contact Lenses

Contact lenses are worn directly on the cornea of the eye. Like eyeglasses, contact lenses help to correct refractive errors and perform this function by adding or subtracting focusing power to the eye's cornea and lens.

About Contact Lenses

Almost 36 million Americans wear contact lenses, and a majority wears daily wear soft lenses. Currently, there are four types of contact lenses in use:

• The soft, water-absorbing lens
• The rigid, gas-permeable lens
• Other rigid lenses
• Other flexible, non-water absorbing lenses

A Contact Lens Prescription

The prescription for contact lenses includes more information than what is available on the prescription for eyeglasses. Special measurements will need to be taken of the curvature of the eye. In addition, the physician will determine if the eyes are too dry for contact lenses and if there are any corneal problems that may prevent a person from wearing contact lenses. Trial lenses are usually tested on the eyes for a period of time to ensure proper fit.

The contact lens prescription usually includes the following information:

• Contact lens power (measured in diopters, like eyeglasses)
• Contact lens base curve
• Diameter of the lens

Unlike eyeglass prescriptions, eye care specialists are not required by federal law to give you a copy of your contact lens specifications. Although, many eye care specialists will give you a copy if you request one.