Introduction to eyeglasses lens design

Many people need only a single powered lens to correct nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Single powered lenses are used for both full-time wear and part-time use, like reading glasses. (Some reading glasses are half-frame, to enable the wearer to see over the top for distance viewing.) Single powered lenses can be made for very strong prescriptions, but they are only designed to deal with one vision deficiency. People with multiple prescriptions will require multifocal lenses.

Bifocals are two-part lenses; the top part is for your distance prescription, the bottom for your reading prescription, and a line separates the two.

Trifocals have three segments and two lines, with a middle segment inserted for the mid-range vision you need to see the dashboard of your car, or a picture you are painting.

There are several designs for multifocal lenses:

* Flat top lenses are the easiest to use because the reading area of the lens is a clearly defined semicircle at the bottom. You always know exactly where to look when you read.


* In a Franklin-style lens, the dividing line runs all the way across, and the reading area is the entire width of the bottom of the lens. These lenses provide the widest field of vision for close work. (Franklin-style lenses are frequently used for trifocals.)


* Progressive lenses have no line at all -- just a gradual transition between the various sections of the lens. They are the most flexible, but distortion around the reading area can be distracting. Newer progressive lens designs reduce or eliminate distortion.


* Aspheric lenses are thinner than conventional stronger lenses, and give better vision when looking to the side, from the center of the field of vision. This design is good for people with strong corrections because they are more attractive and lighter than conventional high-powered, thick lenses.

All multifocals take some getting used to. Until you learn what part of the lens to look through, move your head from side to side when you’re reading or look through the top part of the glasses when approaching steps. Your ophthalmologist may have additional tips to help you adjust.