I have only heard about single vision and bifocal eyeglasses? bifocals have two focal points, one for far and the other one for near, want are trifocal leses? Do they have a special focal point for looking into the sky?

This is my first time to know this kind of lenses, interested, so, what's the advantages of trifocal lenses compared to bifocal ones? Just an other focal point?

Trifocal glasses are invented to solve certain problems, such as myopia, hyperopia, or even amblyopia. Lenses of trifocal are devided into three segment dirstrics. Wearers can see clearly with these three powerzones. And sometimes, trifocal glasses will do more help for computer work, reading newspaper and so on. People who are over the age of 50 are really in need of such glasses. If you want to buy one, cheapprescriptionglasses.net is a good place for you to buy one.