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Trifocals are lenses for eyeglasses with three levels of prescription. Similar to bifocals, trifocals add a third section for correction. They are designed to provide clear vision for near use and use at a distance, as well as intermediate or mid-range use.
Typically, the near and mid-range prescriptions are included in an area that appears as a semi-circle toward the bottom of the lens. That semi-circle includes two horizontal lines to visibly separate the three prescriptions in trifocals. The near range of correction is placed nearest to the bottom of the lens. It is used when the wearer needs to view close work such as reading or hand sewing.
The intermediate or mid-range is toward the center of the lens. It is used when the individual needs to focus on something about an arm's length away. This mid-range segment is often the narrowest band of correction in trifocals. However, it is possible for the intermediate area of the lens to be made larger. This is particularly helpful to those who spend a great deal of time using a computer.
Nearly anything beyond an arm's length requires the wearer to use the topmost section of the trifocals for distance viewing. This topmost section may also contain a prescription to correct other deficiencies such as nearsightedness or farsightedness. Farsightedness is sometimes confused with presbyopia, but in fact, the two are separate conditions.
Trifocals are generally prescribed for individuals with advanced presbyopia. Presbyopia, from the Latin words for "aging eyes," is a normal loss of visual accommodation. As individuals move into their 40s, most begin to notice problems seeing near objects – especially small print. While presbyopia cannot be prevented, it can be treated with refractive surgery or contact lenses, as well as glasses with corrective lenses.
Trifocals offer the wearer the benefits of three corrective prescriptions without the need for three separate pairs of glasses. For those who want the benefits of trifocals without the telltale lines, there are progressive lenses. Progressive lenses may also be called progressive add(ition) lenses or PALs, progressive power lenses, graduated lenses, and varifocal lenses. These terms may be used to describe “no-line” bifocals as well.