This history of glasses lenses can be dated back to more than eight centuries ago. However, those lenses we often use till now are almost single vision and the lenses with multiple focuses did not come into being until the emergence of bifocals. It was the 18th century that witnessed the appearance of the first bifocal lenses. And who is the first bifocal lens inventor? Till now, the universally recognized inventor is Benjamin Franklin, one of the most reputable state men in then US. He found it was impossible for him to read or see other objects with ordinary single vision lenses (he may have both Presbyopia and near vision problems at the same time). He then tried many methods to solve those problems by overlapping two different lenses. And the results prove to be practical.
Theoretically, multi-focal lenses, also called varifocal lenses, have more than one vision areas comparing to ordinary lenses. Bifocals listed above are just such typical example, with two focuses. In fact, there are still some other multi-focal lenses that can be found in optical stores, in addition to bifocals.
Trifocals, also called trifocal lenses, are much advanced products than bifocals. They have three vision areas in the same lenses, namely, near, intermediate and far vision areas. They can provide wearers with much greater comfort while they have to look at objects of different distance.
However, there is a disadvantage in both bifocals and trifocals- there is an evident division line among each vision areas. Because eyes have to endure the pain of transmitting from one area to another, some wearers can not withstand such radical vision changes. Therefore, much advanced lenses are needed.
Therefore, the coming of progressive lenses has satisfied people’s demands. Those lenses are really multi-focal, for the transition among any vision area is very natural and no aforesaid lines can be detected on them. So, wearers of such lenses will never suffer from pain caused by radical vision changes. And the result is more and more people love to wear those lenses. Moreover, progressive lenses often come together with bifocals and trifocals.
Multi-focal lenses are always in the process of evolution, through which more and more achievements are scored.