Lenses are usually classified into one of four categories: single vision lenses, bifocal lenses, trifocal lenses, or multifocal lenses.
Single vision lenses have the same focal power throughout and can be used to correct nearsightedness, farsightedness, astigmatism, or a combination of these disorders. Most people who wear glasses before the age of 40 have single vision lenses.
Bifocal lenses have two parts: the upper part normally used for distance vision and the lower part used for near-vision tasks such as reading. Many people after reaching age 45 develop a condition called presbyopia, which is deterioration in the ability of the eye’s natural lens to expand or contract in order to focus on close objects. People with presbyopia need a special lens for reading and may need a different lens for seeing in the distance. To avoid using two pairs of spectacles, the bifocal was divided into two portions. Upper and lower segments. Segments which can take the form of one of four shapes. Namely the flat top, round, rectangular and executive types.
Trifocal lenses have three different focus areas: the top for distance vision, the centre for intermediate vision, and the bottom for near vision. Although bifocal lenses usually are designed so that the line of division is at the level of the lower eyelid, trifocals are fitted higher, with the top line of the middle region at the level of the pupil.
Multifocal literally means “having more than one focus,” so bifocal and trifocal lenses are actually types of multifocal lenses. In addition to bifocal and trifocal lenses, the other multifocal is a progressive addition (no-line bifocal) lens that increases in power from top to bottom. Progressive lenses have no clear dividing lines as the focus changes from bottom to top. They have become popular in recent years because they look like single vision glasses.