Encyclopedia - Presbyopia

How does the eye see?
What corrective lenses help with this condition?

Presbyopia is the condition that exists when the natural crystalline lens of the eye loses some of its ability to change shape in order to focus on near objects. Sometime after the age of 40, most people eventually develop presbyopia, usually signaled by a need for reading glasses or bifocal lenses.

How does the eye see?

The eye functions much like a camera with two lenses. The first lens is the cornea, a clear membrane that covers the front of the eye. The second lens is the eye's natural crystalline lens, which is located behind the pupil. The cornea is responsible for about 70 percent of the eye's focusing power, while the natural lens "fine-tunes" the image before it is focused on the retina at the back of the eye. The natural lens accomplishes this fine-tuning function by changing shape to accommodate both near objects and those that are further away. Muscles called ciliary muscles are attached to the lens and are responsible for its ability to change shape. (See Anatomy of the Eye.)

People who are nearsighted may not need corrective lenses for reading as soon as other people do, because their eyes naturally focus more easily on objects that are close. Even when the crystalline lens in a nearsighted eye loses some of its flexibility, the flatter cornea compensates and may continue to offer sharp close-up vision.

What corrective lenses help with this condition?

Because presbyopia affects near vision, the usual correction is with reading glasses or with bifocals for those who also require distance vision correction. Several innovations in bifocal eyeglasses and contact lenses have occurred in recent years that allow presbyopes (those people with presbyopia) to see better, look better, and be more comfortable. Progressive or "no-line" multifocal eyeglasses graduate from distance to reading power without the noticeable lines that exist in standard bifocals or trifocals. Although progressive lenses may require a greater period of adjustment, these are the most versatile of all multifocal designs because of the continuous range of focus. Several versions of multifocal contact lenses are also now available that provide near, intermediate, and distance vision in one contact lens.