Glasses and contact lenses correct refractive errors by adding or subtracting focusing power to your cornea and lens. The power needed to focus images directly on your retina is measured in diopters. This measurement is also known as your eyeglass prescription.

Glasses and contact lenses correct refractive errors by adding or subtracting focusing power to your cornea and lens. The power needed to focus images directly on your retina is measured in diopters. This measurement is also known as your eyeglass prescription.

If you have myopia

, your cornea and lens have too much focusing power, bending light rays to meet at a point in front of the retina. Glasses and contacts compensate for this condition by subtracting power from the eye's natural focus and allowing light rays to focus further back on the retina. If you have myopia, your prescription will be negative, for example, -4.25 diopters.

If you have hyperopia

, glasses and contacts add focusing power, causing light rays to bend more as they enter the eye. This process moves the focal point back to the retina, allowing for clear vision. If you have hyperopia, your prescription will be positive, for example, +4.25 diopters.

If you have astigmatism, the shape of the glass lens compensates for the uneven corneal curve and focuses the light rays to a single point on the retina.

Lens power is measured in units called diopters. Diopters are based on the extent light rays passing through the lens will be bent. As the power of the lens increases, so does the thickness of the lens. There are three different types of lenses:

• Convex lenses are thicker in the center than at the edges, like a magnifying glass. Light rays are gathered together towards a central point. Convex lenses are used in glasses for farsighted (hyperopic) eyes that can't bend light rays as much as they need to. Convex lenses are indicated with a plus (+) symbol on prescriptions.
  

• Concave lenses are thinner at the center than at the edges and spread light rays apart. These lenses are used for eyes that are nearsighted (myopic). Concave lenses are indicated with a minus (-) symbol.
  

• Cylindrical lenses are curved more in one direction than the other. To tell if your lenses are cylindrical, hold your glasses at arm's length and sight a straight line through the lens. Rotate the glasses clockwise and counterclockwise. If the line bends, it's a cylindrical lens. Cylindrical lenses, used for astigmatism, are usually part of a prescription for near- or farsightedness.
  

This means the patient’s right eye has 1 ¼ diopters of nearsightedness with 2 1/2 diopters of astigmatism. The axis refers to the orientation of the cylindrical area of the lens. The axis can be anywhere from 1 to 180 degrees, with 90 being the vertical meridians. The left eye has ¾ diopters of nearsightedness, 2 ¼ diopters of astigmatism, axis 90.
Bifocal prescriptions are indicated with numbers such as the "+1.50 add" above. This number indicates the strength of the lens. This patient will need 1 ½ diopters of power for reading.