Single Vision LensesA single Vision lens can be used to correct either farsightedness or nearsightedness. Because there is only a single correction in the lens, there is no difference in power throughout the lens.
Multi-focal LensesThere are several different options within the multi-focal category.
Bifocals - A bifocal has a visible segment added on to the front of the lens, allowing for two different vision corrections. A flat-top is the most commonly dispensed bifocal in the US. The addition, or reading power of a bifocal may be available in different sizes also. For example. a flat-top may be available in a 28mm or 35mm segment. This refers to the width of the segment.
Trifocals - A trifocal has a visible segment added on the front of the lens that is separated into two sections: the near and intermediate, allowing for three different corrections. As with bifocals, the segment may be available in different sizes. A "7 by 28 trifocal", written as 7x28, has a segment that is 28mm wide and the intermediate portion of that segment is 7mm high. While the 7x28 is the most common sized used, another common size of trifocal is an 8x35.
Progressives - Progressive lenses (sometimes referred to as no line, invisible or PAL) provides a continuous change in power as the eye moves within the lens from the distance viewing area to the reading area. Progressive lens use is growing faster than any other multi-focal style because the allow for multiple corrections with no visible segment, increasing the cosmetic appeal of the lens. Each progressive lens design can be identified by the engraved (semi-visble) markings on the front surface.