John Isaac Hawkins invented the trifocal lens in 1827 in an effort to solve the problem of having no intermediate viewing area in the bifocal lens.
Trifocals are lenses with three distinct corrective powers. Trifocals correct for distance, intermediate (arm’s length) and near vision. Trifocals are commonly prescribed to advanced presbyopes who require a correction for 2 diopters or more of reading addition. The intermediate portion of the trifocal lens is typically half of the addition power.
Therefore, a person with a -5.00 diopter distance power with a +3.00 add power would have net reading power of -2.00 diopters and an intermediate power of -3.50 diopters.
The most common trifocal today is the 7×28 trifocal which has a straight line across the top with another straight line 7mm below that (this is the intermediate portion of the addition) and a 28mm radius . There are several varieties of straight top trifocals available today including: 7×25, 7×28, 8×35, and the Executive Trifocal which runs the complete width of the lens.
There are specialty trifocals available that are often suggested for those who spend a great deal of time on the computer and for those 50 years an older who are having a harder time accommodating. As mentioned above the standard intermediate power for the trifocal is 50% of the total add power. This line of trifocals (called Acclaim 61) has an intermediate power that is 61% of the total add power. These trifocals come in 8×34, 10×35, 12×35 and 14×35. Material options for lined trifocals is more limited than with other lens designs.
Some people are reluctant to try trifocals because they are concerned about the appearance of the lenses or are worried that trifocals will be difficult to get used to, since the top line of the intermediate zone is close to the center of the lens. But in fact, most people who try them are very pleased with the added range of vision trifocals provide (compared with bifocals) and the wide field of view they offer for computer use. Pricing