Eyeglasses, Sunglasses, and Magnifiers (cont.)

Some people will require multiple distances or focal lengths for a wide variety of tasks, including near and intermediate tasks. Near tasks include reading, arts and crafts, fingernail trimming or model airplane building, while intermediate tasks include computer use, knitting, shaving at the bathroom mirror, or reading piano music while performing, for example. When presbyopia advances, our eyes lose the ability to see intermediate as well as near items. For these special needs, a trifocal lens may be required. This allows for clear distance, intermediate, and near vision. The trifocal lens is generally placed in the same location as the bifocal, with a smaller segment closer to the center of the lens.

A further advance is the variable focus lens, in which the lines between the bifocal and the distance lens are eliminated, providing a gradual transition from near to far, with intermediate therefore in between. Varilux is a common brand name among numerous others. Many people prefer these more expensive lenses because the transition line is so annoying. It may take several weeks to become accustomed to this type of lens. These lenses provide the intermediate distance in the region between the bifocal and the distance segments, thus creating a trifocal capability using a simple subtle tilt of the head as well.

In some cases, customers may also be asked for the "pupillary" distance. This is actually the interpupillary distance, the distance between the pupils of the eyes. The pupils, although black in appearance, actually represent the round clear optical space defined by the center of the colored brown or blue iris. This distance is usually measured in millimeters.

Doctors do not recommend nonprescription glasses as a permanent solution to significant eye problems because they usually are not as accurate as prescription lenses. In addition, the use of OTC glasses could lead to the postponement of an eye examination that could reveal a serious underlying problem such as glaucoma, cataract, or macular degeneration. However, OTC glasses are okay to keep as "backups" for times when prescription glasses have been misplaced or are being repaired. Be certain to follow the recommendation of your eye care provider, an ophthalmologist or optometrist, before relying upon OTC readers.

Source: MedicineNet.com
http://www.medicinenet.com/eyeglasses_sunglasses_and_magnifiers/article.htm