Bifocals are used for correction of presbyopia. most ancient type of lenses invented by Benjamin Franklin for reading. As they two focal length one for distance and one for reading that's why they are called bifocals
There are two types of bifocal lens as shown here one is
1) Round seg
2) Flat top bifocal.
Flat top bifocals are most optical friendly and scientific types of bifocals but it lack cosmetic appeal and look.