Multifocal eyeglass lenses are glasses that contain two or more levels of lens powers. The purpose of this is to allow the user to focus on objects at different distances, depending on which part of the glass they are looking through.
The condition that leads to a reduced ability to focus on objects is called presbyopia. Sufferers of presbyopia are commonly prescribed varying types of bifocals depending on what they need help in focusing on. One common type of bifocal lens is the flat-top or D-segment bifocal lens, which can also at times be referred to as a D-seg or straight-top. Other types of bifocal lens include round-segment bifocal lenses, Franklin or Executive bifocal lenses, which are sometimes called the Franklin bifocal lens.
Bifocals contain two types of lens power. A more complex arrangement is the trifocal which has three. Progressive multifocal lenses are lenses that gradually change in power from the top to the bottom part of the lens. As they technically contain a wide variety of specific lens powers they are called multifocal lenses. Multifocal lenses usually are most likely to be prescribed for adults over the age of 42. However, there are some specific conditions that will lead to bifocals being prescribed for people under the age of 42. These conditions include eye teaming or focusing problems that are causing the person some form of eye strain in situations such as reading or looking at small objects close to the viewer. For these kind of conditions, the bottom portion of the bifocal lens helps the user to attain a clearer level of focus on objects that are close to them in physical terms. The basic principle of bifocals is for the small lower area of the lens to help with your near vision (vision of objects that are near to you) while the upper part of the lens is for distance vision.
The lens segment (which can technically be referred to as a ‘seg’ that is used for near-vision correction comes in a variety of shapes. It can be half-moon (or a flat-top), a straight-top (D-segment), a round segment, a rectangular (or ribbon) area or the entire lower half of a bifocal lens regardless of the shape of the lens, in which case it can be called a Franklin, Executive or E-style. The bifocal part of the lens is most accurate when you are holding an object or book within about 19 inches of your eyes.
The design of bifocals is an important part of how they function. The line separating the near-vision area of the lens from the distance-vision area tends to be designed to sit at about the level of the bottom of the user’s eyes. So it is only when you glance slightly downwards that you are looking through the near-vision portion of the lens.
As well as the standard bifocal lenses, you can get specially designed bifocal lenses for particular occupations or tasks. Some golfers use specially designed bifocal lenses with a particular layout of the lenses that helps them to see clearly while they are playing golf. The ‘Double-D’ lens is a lens that has two flat-top segments for near vision. One of the flat-top segments is at the top of the lens, while the other is at the bottom of the lens. The centre of the Double-D lens is equivalent to the usual upper half of a bifocal lens, as it is adjusted for distance vision. The Double-D multifocal lens is used by people who might need to see things that are close to them clearly when they are looking slightly upwards as well as slightly downwards. Technically, the Double-D is either a trifocal or a bifocal lens depending on how you choose to define bifocal and trifocal and on whether the two near-vision flat-top segments are calibrated to an identical level of lens power. The Double-D multifocal can be used by car mechanics, who need to look down but also sometimes to look up at vehicles on a ramp, or by librarians who may need to read a book or to look at high shelves. Another special-purpose trifocal lens is the multifocal lens known as the E-D triofcal.