Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arm's length), and near vision. They are mostly used by people with advanced presbyopia who have been prescribed 2 diopters or more of reading addition. The intermediate addition is normally half the reading addition. So, for someone with a distance prescription of -4 diopters and a reading addition of +3, the reading portion of their trifocals would have a net power of -1, and the intermediate segment would be -2.5 diopters.
Trifocal lenses are made in similar styles to bifocals, but with an additional segment for intermediate vision above the reading section. A common style is the 7x28 flat-top or D-shaped segment, 28 mm wide, with a 7 mm high intermediate segment. Larger intermediate segments are available, and are particularly useful for people who spend a lot of time using computers.
Trifocals are becoming rarer as more people choose to wear progressive lenses.
----------------------from en.wikipedia.org
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