A 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.
There are several different options within the multi-focal category.
- 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.
- 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.
- 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.
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