The Magnification Needs for Different Visions

The most common types of vision problems are refraction errors. Refraction is simply the way light bends as it passes through the eye and is converted into signals that the brain interprets as specific images. If the light does not focus directly on the retina, then vision suffers. A diopter is the measurement of how much focusing power is added or subtracted by a lens. Each of the four common refraction errors require a specific type of magnification and correction.

Nearsightedness

• Nearsightedness is also called myopia. The light rays passing through the eye focus in front of the retina. Nearsighted people often have trouble seeing distant objects clearly but can focus on close items. Lenses for myopia are concave -- thin in the middle and thicker at the edges. These lenses are always measured in minus diopters, which subtracts focusing power and focuses the light backward and onto the retina. A higher diopter number, such as minus 6, means a more severe correction than a minus 2.

Farsightedness

• Farsightedness -- also called hyperopia -- is the opposite of nearsightedness. Farsighted people can see distant objects clearly, but closer objects appear blurry. Farsightedness occurs when the light rays are focused behind the retina. Lenses for farsightedness are convex -- thin at the edges and thicker in the middle. Lenses for hyperopia are measured in positive diopters. This adds focusing power, allowing for better up-close vision. A higher diopter number -- this time positive -- means the refractive error is more severe.

Astigmatism

• Astigmatism causes the light to be unevenly focused on the retina. Generally, the shape of the cornea has changed from a ball into a more oblong shape. Images appear blurry and elongated. Astigmatism requires a cylindrical lens that curves more to one side or the other. The correction may require either plus or minus diopters, depending on the specific shape of the eye. More severe correction will be reflected by a higher diopter number.

Presbyopia

• Presbyopia is an age-related refractive error. The lens of the eye hardens with age, and this prevents the eye from being able to focus the light onto the retina. People with presbyopia are unable to focus on small print or objects that are too close to the eyes. Presbyopia can occur in addition to other refractive errors. People with presbyopia generally cope by wearing either bifocals or reading glasses. These lenses are relatively low power, commonly ranging from positive .25 to 2.5 diopters.