Bifocal glasses become necessary after the appearance of the so-called "age-related farsightedness", an eye condition that appears with age, which is scientifically called "presbyopia". Their lenses combine two tricks: to look into the distance and to look close, for example, to read.
Presbyopia is a change in vision that has age-related causes. Therefore, it has a household name "age farsightedness". At a certain age, about forty years or older, vision deteriorates, especially near - it becomes difficult to read and consider small objects. The reason for this is a decrease in the elasticity of the lens and eye muscles, which, having crossed the age threshold, lose the ability to focus. This problem occurs in all people, even those who have always had excellent vision, as well as those who had problems with myopia. The fact is that we are talking about a completely different nature of the fall in vision than that of myopia (nearsightedness) or hypermetropia (farsightedness), although the consequences of presbyopia are similar to the latter.
Therefore, we need glasses for farsightedness, i.e. "plus". They give us back the ability to read and see small objects clearly up close. Presbyopia begins with limited vision at close range, but then moves on to medium vision, where the focus will be different – this is how the second focus appears in glasses. And if you have myopia, you need another one to see clearly over long distances. In order not to have two or three glasses to look at different distances, and there are glasses that combine several tricks. They are called bifocal (with two focuses) or progressive (with two or more).
Glasses that allow you to see both near and far are called bifocals. This is possible due to the fact that spectacle lenses are divided into two optical zones with different focal lengths: one, for distance vision, is located at the top, the other is at the bottom. Through the lower part, which just corrects presbyopia, you look at what is close, small objects, read. It's like having normal reading glasses, you lower them and look into the distance over the glasses. Only these do not need to be lowered, the ability to “look over” is built into the glasses themselves. Due to the two focuses inside one lens, these glasses are called "bifocals".
Features to consider:
Glasses with progressive lenses ("progressive glasses") are similar to bifocals, but devoid of their disadvantages. They are more finely tuned, without sharp transitions, with several smoothly changing focuses, and you can see them at near, far and medium distances. Therefore, they are also called "multifocal". In addition, they look like ordinary glasses. And since bifocals clearly mark age after 40, the absence of such an indication of age is a pleasant bonus of progressive glasses.
As for the difference between the zones for close and far distances, it is still 2-3 diopters, but there is a "progression corridor" between these segments, which allows you to see well at an average distance. In this transition zone, the optical power changes gradually, so the gaze moves smoothly without stuttering. The zone, which is called the “corridor of progression”, is cleverly planned: it goes from top to bottom and gradually narrows towards the nose, which is physiological when reading.
There are lens models with a narrow progression corridor, and they require you to turn your head to view an object at an average distance – in order for it to be in the right zone. This problem is being worked on, and there are already glasses where the intermediate zone is wider and this is not required.
Glasses with progressive lenses that correct presbyopia are made only to order, because everyone has different needs and vision parameters. It is necessary to take into account all the factors, and there are about twenty of them. After all the indicators have been measured on special equipment, which is usually equipped with an ophthalmologist's office, a suitable frame is selected, and special markings are applied to the lenses. When the spectacles are ready, fitting under the supervision of a doctor follows, and then during the adaptation, all comments must be handled. The better the vision parameters were taken, the less comments thereafter. most of the time they don't.
Contact optical salons, where you can check your eyesight, choose a suitable frame and order glasses.