The oldest and the simplest version of prescription glasses are the single-vision glasses. The entire lens carries the same power, and its most common example is the reading glasses. Though simple to use and light-weight, these glasses are not much effective when it comes to cure vision for people who suffer from both hyperopia and myopia.
Bifocal glasses are generally
prescription glasses with lenses which have two different optical powers. These bifocal glasses are prescribed to people suffering from presbyopia and who also require a correction for myopia, hyperopia and/or astigmatism.
Bifocal glasses are mostly crafted by molding a reading segment, used for near vision, into the primary lenses. Since these lenses are available with the reading segments in diverse shapes and sizes, the choices for buyers have also increased. One of the most popular ones is the D-segment which is generally 28 mm wide. Though these D segment bifocal lenses offer better viewing qualities, an increasing number of users are going for progressive bifocal lenses.
Varifocal glasses, also known as the progressive glasses, graduated glasses and no-lines bifocals are generally used to correct presbyopia and have become one of the most used glasses for prescription as well as
designer glasses. The name comes from the fact that the glasses have different power at different levels of the lens. At the top of the lens, the power is minimum, which gradually increases in the middle level and finally, the bottom part of the lens has the maximum power.
The greatest advantage of
varifocal glasses is the ability to look at objects at varying distances with just a slight tilt of head. As the different levels have different powers, the top part can be used to view objects at a distance, the middle one at the objects at an intermediate distance and the bottom part while working on nearby objects, like reading a book. They also avoid the discontinuities like image - jumps which occur in the visual fields created by bifocal and trifocal glasses. Commercially, varifocal glasses are quite popular too, because of the absence of any line on the lens.
