Bifocals are special multi-focal glasses. They are suitable for people who have two refractive errors.
What are bifocals?
Bifocals eliminate the need to switch between distance and reading glasses. With the help of bifocals, two different refractive errors can be corrected at the same time. The Latin term ‘bifocal’ means ‘two’ (‘bi’) and ‘focal point’ (‘focal’). Thus, a bifocal lens achieves two different optical effects and can be used for different distances. In most cases, bifocals are used to correct nearsightedness and age-related farsightedness. Both refractive errors have different causes. If nearsightedness exists, sharp vision into the distance is limited, which is why distance glasses are needed. If, on the other hand, presbyopia is present, it is no longer possible to see objects sharply that are close to the eye. Reading glasses are used to correct this problem. Bifocals were invented as early as 1770 by Benjamin Franklin (1706-1790). Franklin suffered from always having to exchange his distance glasses for reading glasses. Finally, he conceived the idea of attaching a corresponding optical effect to each side of the glasses. This lens was called a Franklin lens. Today, bifocal lenses are manufactured using two different types of glass. In this process, the melting of a glass part (which has a higher refractive index) into the carrier glass, which is kept in the distance power, takes place. In this way, a smooth surface is created on which the transition between the near part and the distance part cannot be felt.
Shapes, types and types
Bifocals belong to the group of multifocal glasses. In these, different vision corrections are combined within a single lens. Thus, they have more than one focal point. That is why they are also called multifocals. Trifocals are a variant of bifocals. They are also used to correct two refractive errors. Just as with bifocals, the main lens acts as a distance lens to correct myopia. There is also a framed lens that acts like reading glasses and corrects presbyopia. In addition, trifocals also have a zone for intermediate visual distances. In today’s world, however, bifocals and trifocals are considered rather outdated. Instead, more modern varifocals are used. With these glasses, the transition between the different refraction angles is smooth or flowing. The equipment of varifocals consists of three zones. Thus, the upper zone is used to detect objects from a distance and the lower zone is used to correct near vision. The gliding middle section is located in between and is considered optimal for a special intermediate distance. However, varifocals are less suitable for use in front of a computer monitor.
Structure and operation
Bifocals are composed of two lenses that have different refractive powers. Inside the main lens, which is used to correct myopia, is a smaller lens. This is specially made for optimal near vision. Both lens glasses have a different curvature. Their refractive power is also different. This results in the formation of a dividing line between the lenses, which can be clearly seen. It is considered a characteristic of bifocals. However, many people classify this edge as annoying. For the material of bifocals, either glass or plastic is used. In bifocals made of glass, the manufacturer cuts an opening. The reading glasses are then melted into this opening. In the case of bifocals made of plastic, the glasses are made in one piece. The reading part of bifocals is made in different widths. Common measurements are 25, 28 and 40 millimeters. The wider the measurement, the more expensive the glasses. Any frame can be used for bifocals. With the help of bifocals, the annoying exchange between distance glasses and reading glasses can be avoided, as they correct both nearsightedness and farsightedness.However, the wearer of the special glasses often has to get used to the fact that only part of the lens serves him for a sharp image. Thus, with a lowered gaze, objects in the distance on the ground, can sometimes be perceived only blurred.
Medical and health benefits
Bifocals are useful for people who suffer from both nearsightedness and age-related farsightedness, and therefore rely on two different pairs of glasses. With bifocals, both refractive errors can be corrected. It is then no longer necessary to switch between two pairs of glasses. Bifocals are also used therapeutically to treat hypoaccommodative convergence excess. The same is true for other strabismus disorders that are accommodative and have extensive convergent near angle. However, in some cases, bifocals can cause problems. For example, the human eye is strained by the alternation between distance and near vision, but this is not normally tiring. However, distortions and dizziness often occur, especially during the acclimatization phase. Since the bifocal spectacle wearer strains and concentrates excessively while seeing, eye tears and headaches are also not uncommon. For this reason, after purchasing bifocals, it is recommended to take it easy and wear the glasses on a weekend or take a break for a day in between. In order for the bifocal glasses to develop their positive effects, it is necessary to determine the wearer’s visual acuity precisely in advance. In addition, the lenses must be accurately centered.
