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Why You Need Reading Glasses: Presbyopia Explained

The need to begin wearing reading glasses regularly when reaching age 35 - 40 is due to presbyopia (pres-be-O-pe-uh), the medical term for a condition that mimics farsightedness. It's virtually a universal condition, and unfortunately, no refractive surgical procedures can currently correct presbyopia.

In fact, middle aged colonial Americans with presbyopia imported spectacles from Europe. Spectacles were mainly for the affluent and literate colonists, who required a valuable and treasured appliance, and they cost as much as $200 in the early 1700's. Benjamin Franklin had presbyopia. Perhaps that is why he is identified by his spectacles, and later the bifocals he invented.

Here is a technical explanation of presbyopia:

When your eye focuses on a distant object, the ciliary muscle surrounding the lens relaxes; the lens thins and flattens. As you focus at close range, the ciliary muscle contracts. The curvature of the lens is increased, and the lens thickens. At about age 35, the problem of focusing becomes noticeably symptomatic because your lenses will have thickened naturally, and they will be harder and less pliable. The ciliary muscles in your eyes also will have become weakened, will have lost tone and will no longer be able to shape the lenses properly to focus on near objects. This is because the image would be in focus behind, rather than on the retina, thus causing blurring of close-range vision.

In simple language, presbyopia is an inevitable adjunct to aging, and is due to the natural stiffening of the lens in the eye, with the consequence that your eyes are prevented from focusing on close objects. While you still can see distant objects clearly, you cannot refocus on near objects. Adding an extra lens in the form of reading glasses will serve to focus close objects properly on your retina. 
Reading glasses are available without a prescription, and are intended as an aid if you have blurred vision at close distances. However, they are not intended to replace prescribed corrective eye wear, nor to replace professional eye examinations and care.

Typically, reading glasses can be safely, self-prescribed by you, the user, with the help of a simple eye chart.
The level of magnification in the lenses of reading glasses is called "diopter strength". A very weak magnification would be found in reading glasses with a diopter strength of 1.00. Reading glasses are often offered for sale in diopter strengths that start with 1.50, and stronger lenses are offered in diopter strengths that increase by a factor of .25 (e.g., 1.50; 1.75; 2.0; 2.25; 2.50; etc). Presbyopia is progressive, and therefore, the strength of magnification in your reading glasses will probably have to be increased every year or two. Moreover, it is not uncommon to need a second pair of reading glasses with greater magnification for stressful times, as stress can intensify the near vision blurring caused by presbyopia.

For more information about diopter strength, please click here.