Tri-focal Lenses
Trifocal Lenses consist of three segments. The upper segment allows you to see the objects clearly which are in a distant from the eye, the bottom is used during reading and close work, and the middle - for the consideration of objects at a middle distance from the eyes. Sometimes musicians use the middle segment lenses for reading notes during the concert.
You can literally say that Trifocal Lenses - it is like three pairs of glasses in one.
Why do you need them?
With age, the ability of the eye to focus decreases, and it may happen because you can not see everything clearly at all distances. Options of the top lens allows you to see everything at a distance of 1,5 meters away, the segment "read" lets you to see everything in the immediate vicinity. This leaves space in the space of half a meter and a half in which to distinguish anything becomes quite difficult. A third segment trifocal spectacle lens can provide you to see everything clearly and distinctly and at such distances. A typical costumer of these points can be a computer operator or programmer who needs to distinguish the letters on the screen. The difference between the individual segments in conventional bifocal glasses and trifocal is large, so that the transition from one segment to another can be quite sharp. In recent years increasing popularity of a particular type of lens, where the transition from one segment to another is carried out as it gradually - from the "given" to "near."
Such lenses are called "progressive complementary." Theoretically, in this case, the clarity of view can be achieved when you view through a particular portion of the lens, in practice it needs some adaptation, because the size of the middle part and the portion for reading is limited. Additionally, at the intersection of these zones there may be slight distortion. Most people adapt to these lenses for a week and they are usually very happy. In such multifocal glasses the line between the segments is invisible, so they are called "invisible" or "visible". From a cosmetic point of view, they certainly seem preferable. Trifocals double segment designed for those who need a clear, wide field of view in the middle distance, but it is important to see well in close and at long range. Glasses with such lenses are is useful for TV producer because he has to watch on multiple monitors in the same time located in front of and to the sides of the room. A person with such a profession has to be able to read the text on the table and see the other people on the far side of the room.