Eyeglasses correct vision problems, such as nearsightedness,
farsightedness, astigmatism, and presbyopia by focusing light more
appropriately on the retina.
The type of vision problem that you have determines the shape of
the eyeglass lens. For example, a lens that is concave, or curves
inward, is used to correct nearsightedness, while a lens that is
convex, or curves outward, is used to correct farsightedness. To
correct astigmatism, which is caused by distortions in the shape of
the cornea, a cylinder-shaped lens is used. Presbyopia requires
bifocal or multifocal lenses.
What are multifocal lenses?
People who have more than one vision problem often need glasses
with multifocal lenses. Multifocal lenses, bifocals, trifocals, or
progressive lenses are lenses that contain two or more
vision-correcting prescriptions.
- Bifocals: Bifocals are the most common type of
multifocal lens. The lens is split into two sections; the upper
part is for distance vision and the lower part for near vision.
They are usually prescribed for people over the age of 40 whose
focusing ability has declined because of presbyopia.
- Trifocals: Trifocals are simply bifocals with
a third section for people who need help seeing objects that are
within an arm's reach.
- Progressive: Progressive lenses have a
continuous gradient (inclined) lens which focuses progressively
closer as one looks down through the lens.
What types of lenses are available?
In the past, eyeglass lenses were made exclusively of glass;
today, however, most lenses are made of plastic. Plastic lenses are
lighter, do not break as easily as glass lenses, and can be treated
with a filter to keep out ultraviolet light, which can be damaging
to the eyes. However, glass lenses are more resistant to scratches
than plastic ones.
As technology advances so, too, do eyeglass lenses. The
following modern lenses are lighter, thinner, and more
scratch-resistant than the common plastic and glass lenses:
Polycarbonate lenses: These lenses are
impact-resistant and are a good choice for people who regularly
participate in sporting activities, work in a job environment in
which their glasses may be easily scratched or broken, and for
children who may easily drop and scratch their glasses.
Photochromic and tinted lenses: Made from
either glass or plastic, these lenses change from clear to tinted
when exposed to sunlight. This eliminates the need for prescription
sunglasses.
High-index plastic lenses: Designed for people
who require strong prescriptions, these lenses are lighter and
thinner than the standard, thick lenses that may otherwise be
needed.