What is the difference between bifocals, multifocals, and trifocals?

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What is the difference between bifocals, multifocals, and trifocals?

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A. Bifocals are lenses that have two parts – the upper part is used for distance vision, and the lower part is used for such close vision tasks as reading or sewing. Even though bifocals can technically be considered multifocals (because they have more than one focus), the term “multifocal” is generally used interchangeably with the term “trifocal”, because they have three or more segments. In trifocals, the upper part helps view distant objects, the intermediate segment is effective for arms-length vision, and the lower section is for reading-distance vision.