As they age, people often find it increasingly difficult to read or focus on things that are close to them. This is due to a vision condition called presbyopia, a natural part of the aging process that happens to most people. Presbyopia is a loss of flexibility, or hardening, of the eye lens. It happens gradually, over a period of many years. It is not a disease and cannot be prevented or cured. Symptoms become noticeable to people in their early to mid forties.
Most people become aware of presbyopia when they must hold things increasingly farther away to read, or must squint to focus. Print at normal reading distance becomes blurry and unreadable, and close work causes eye fatigue and often headaches. Reading in dim light becomes nearly impossible. These signs are strong indications that it's time to consider bifocals. People who need no distance correction may choose reading glasses, which can be prescription or purchased over the counter.
Bifocal lenses for eyeglasses combine two prescriptions into a single pair of glasses. The top part of the lens addresses problems with distance vision, and the lower part is formulated to correct vision for close-up focus. The section of the lens for close-up focus can represent half the lens area or a very small part of it, like a little circular bubble at the bottom of each lens. Today there are alternatives to bifocals, including trifocals, which have 3 distinct viewing areas, and progressives, which offer a graduated viewing area from near to far with a middle distance area.
Using bifocals can take some getting used to. Some people experience dizziness or blurred vision. If this does not go away in a few days, it may indicate a problem with the glasses that needs adjustment by the doctor. Some self-training is necessary to adjust the head and eyes to the right angles, tilted slightly up for reading and slightly down for distance.
The invention of bifocal glasses is credited to Benjamin Franklin, although there is little hard evidence to support this. In his writing, Franklin often made mention of "my double spectacles," most notably to his friend, philanthropist George Whatley. In a letter dated May 23, 1785, he tells Whatley that he found it troublesome to change from one pair of glasses for normal use to another for reading, so he "had the glasses cut and half of each kind associated in the same circle." To implement this remarkable idea, he enlisted the aid of a Paris optician, H. Sykes, who wrote a letter to Franklin on April 24, 1779, to explain the delay in delivering his new glasses. It seems Sykes broke three pairs while trying to cut them in half as instructed. Later, a design alteration from round to oval was suggested by President Thomas Jefferson because the lenses had a tendency to rotate in the frame, leaving the bifurcation in the wrong place.
As people continue to age, their vision will also continue to change. Regular checkups are recommended to maintain optimal vision correction and eye health.