Bifocal lenses

Upon reaching the age of 40 people begin to have difficulties focusing their vision on the near objects, especially while reading. The accommodative possibilities of the eye start to decrease - appears the so-called presbyopia. 

Bifocal eyeglass lenses have two vision correction zones. The upper part contains a large zone for distant vision, and the bottom part of the lens has an integrated window with the optical power required for the near work. The transition between these two zones is abrupt, with clear boundaries. Such lenses can replace two pairs of glasses - for near and for distance. 

Absence of the middle zone is a drawback of bifocal glasses. Furthermore, 
the boundary between the optical zones in bifocal eyeglass lenses is very noticeable, the reading segment in the bottom of the glass looks like a "window", thus revealing age, and visually increases the lower eyelids and the area under the eyes.

The bifocal lenses are being gradually pushed out of the market by more advanced technology for presbyopia correction - progressive lenses