How Bifocals Work

What are Bifocals?

Bifocals are glasses with two prescriptions placed into one frame. There are several ways bifocal lenses are manufactured, depending on price and appearance. The most expensive lenses are progressive, where there is no discernible difference between the two lenses. This is usually the most desirable lens because they look exactly the same as regular, one-prescription glasses. Less-expensive bifocal lenses contain two visible segments, with the prescription for distance making up the larger part of the lens, and the prescription for reading in a D-shaped sliver or circle on the bottom front of the distance lens. Bifocals can also be made with Ben Franklin-style lenses, where the lenses are horizontally split down the middle, with the distance prescription in the top half and the reading prescription in the bottom half. All bifocal lenses are effective and depend on the personal preference of the patient and the price the patient is able to afford.

Prescribing Bifocals

Bifocals are particularly useful for patients who have presbyopia, which usually occurs in middle age when the eye loses its ability to focus on close objects, according to the Mayo Clinic. Patients with both nearsightedness and presbyopia are generally prescribed bifocals, so they do not have to switch between two pairs of different prescription glasses throughout the day. A patient may need to see a doctor for bifocals if he is holding reading material an arm's length away in order for the text to come into focus. Some patients with advanced presbyopia may need trifocals, which has three prescription segments instead of two. The patient must look into the first segment for reading, the second segment for looking at distances from 18 to 24 inches away, and the third segment for looking at distances over 24 inches away. For golf players, golf bifocals may also be prescribed, which contain a reading prescription in the bottom corner of each lens, closest to the nose. This allows golfers to see the ball clearly with both eyes while they are leaning down over it.

Why Individuals Need Bifocals

For a person with 20/20 vision, objects reflect rays of light that enter the eye properly and land clearly on the retina in the correct position. This allows perfect focus on these objects. But because each individual's retina is curved differently, the light may not always hit the retina in the correct position. For example, people who are nearsighted have rays of light hitting just in front of the retina. People who are farsighted experience rays of light hitting behind the retina. Both problems lead to blurry vision, but in a different spectrum of focus. The lens of the eye and the cornea work to focus the reflection of light, regardless of how near or far it is. If the light reflects directly onto the retina, the object will be in focus. When presbyopia occurs, however, the lens of the eye becomes harder with age, losing its ability to focus, requiring a more direct placement of light in the retina to avoid having to rely on the lens.

How Bifocals Correct Vision

Bifocals usually involve one farsighted prescription and one nearsighted prescription. The farsighted prescription uses a concave lens, which curves in towards the patient. This lens bends light into the center of the retina, allowing far-away objects to come into focus. The nearsighted prescription, usually located at the bottom of the frame, is a convex lens, which curves out away from the patient. This lens refracts light towards the top and bottom of the lens, pushing the focus of the light into the center of the retina, allowing close objects to come into focus. Thus, depending on which prescription the patient looks through, the light will be refracted forward or backward to focus it correctly. The higher the prescription, the more curved each lens will be to allow for greater light refraction.

Wearing Bifocals

Bifocals are placed on the bridge of the nose and are fitted so the glasses do not need to slide up and down in order to work properly. If the patient is sliding his glasses up and down to see clearly, the lenses should be refitted. The eyes drop down onto a page to read, so the natural placement of the reading prescription is below the prescription for distance. Bifocals are often difficult to get used to, and for several weeks the patient may experience nausea and vomiting. As the patient learns to focus on each prescription separately and change his range of vision rather than changing the position of the glasses, he will get used to them within several weeks. For patients who dislike wearing bifocals, laser surgery or bifocal contact lenses are available, which are more expensive options but may make adjusting to both lens prescriptions unnecessary or easier.