What are Bifocal Lenses, Intermediate-distance Lenses and Very-close Distance Lenses?
There are a number of types of spectacles (senior spectacles) available for the correction of presbyopia.The type introduced here is the [progressive lens].
This type of lens has no visible dividing line between the different prescription segments and enables the wearer to see objects in the distance, at close range and in the intermediate distance through a single lens. This type has become the mainstream senior lens.
Although no longer common, another type of lens is the [bifocal (trifocal) lens], a type with visible lines dividing the different prescription areas. Although, in terms of performance, this lens features a very wide range of vision for objects at close range, the eyes have to cross the dividing line between the different prescription areas to look at distant objects, impeding clear vision.In addition, since wearers of spectacles using this kind of lens can easily be identified as sufferers of presbyopia, this type is not very appealing in terms of appearance.
For these reasons, most people nowadays use the [progressive lens] for the correction of presbyopia. Let’s now move on to take a look at the various types of progressive lenses and points to bear in mind when selecting lenses.
*We do not manufacture this type of lens
People with presbyopia using dedicated reading spectacles have to use one pair of spectacles to look at objects in the distance and another to view objects nearby. The multi-focal lens eliminates this bothersome procedure by allowing the wearer to see objects both in the distance and at close quarters through a single lens that features a separate portion for each range. The most common type of multi-focal lens is the bifocal lens which features a single lens for viewing objects at close quarters at the bottom of the lens. In addition, the tri-focal lens features a lens for viewing objects in the intermediate distance as well as the far-sight and near-sight lenses.
The bifocal lens* is comprised of a portion for viewing objects in the distance and a segment for viewing objects at close quarters. Objects in the distance and nearby can be viewed from the lines of vision shown in the illustration.
Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.
Benjamin Franklin is generally credited with the invention of bifocals. Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between George Whatley and John Fenno, editor of The Gazette of the United States, suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought. However the College of Optometrists concluded:
- Unless further evidence emerges all we can say for certain is that Franklin was one of the first people to wear split bifocals and this act of wearing them caused his name to be associated with the type from an early date. This no doubt contributed greatly to their popularisation. The evidence implies, however, that when he sought to order lenses of this type the London opticians were already familiar with them. Other members of Franklin’s circle of British friends may have worn them even earlier, from the 1760s, but it is at best uncertain (and arguably improbable?) that split bifocal lenses had a famous gentleman inventor. 
Since many inventions are developed independently by more than one person, it is possible that the invention of bifocals may have been such a case.
John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Dr. Franklin.
In 1955, Irving Rips of Younger Optics created the first seamless or “invisible” bifocal, a precursor to all progressive lenses.
Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes.The most popular is the D-segment, 28 mm wide. While the D-segment bifocal offers superior optics, an increasing number of people opt for progressive bifocal lenses.
Bifocals can cause headaches and even dizziness in some users. Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head. This trouble is mitigated by the use of trifocal lenses or by the use of monofocal lenses for computer users.
In an interesting legal case reported in the UK in 1969, the plaintiff’s ability to use bifocals was impaired by accident.
Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a liquid crystal layer sandwiched between two glass substrates.
Bifocals in the animal world
The aquatic larval stage of the diving beetle Thermonectus marmoratus has, in its principal eyes, two retinas and two distinct focal planes that are substantially separated (in the manner of bifocals) to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly mosquito larvae. This is the first ever recorded use of bifocal technology in the animal world.
Types of Progressive Lenses
【1】Bifocal Type (Lens for normal daily use)
This is a very convenient type of lens that allows the wearer to focus on objects both at close range and in the distance through a single lens and eliminates the irksome need to frequently remove spectacles. This type of lens is designed to enable viewing of objects in the distance, at close range and in the intermediate distance with corresponding changes in power for each distance. Initially, these variations in power can, however, give the wearer uncomfortable feelings of swaying or distortion, or a sense of a narrow field of vision. (Almost all users quickly adapt and become accustomed to the variations in power.) Since these problematic sensations of swaying, distortion or narrow field of vision become more pronounced the greater the power of the lens, it is important to start using this type of lens from the 40’s before the additional power (presbyopic power) becomes too great. Once the additional power increases to a certain degree, it may be difficult to become accustomed to the lens and the process of adjustment may take some time.
【2】Long-distance bias type (Ideal for driving)
This type of lens offers a wide field of forward vision and allows the user to see objects at close range without changing spectacles, making it the ideal choice for driving. This lens gives the wearer a wide range of long-distance vision and, at the same time, allows viewing of objects at close range such as meters, navigation screens and expressway tickets. This type can also be used for sports such as golf.
【3】Intermediate-distance bias type (For indoor and interior use)
Developed for indoor and interior use, this type of lens allows the wearer to view objects from a distance of about 3 – 4m ahead to objects at close range without removing the spectacles. Spectacles using this type of lens are, therefore, ideal for homemakers, for example, when watching TV, cleaning, cooking or reading. Moreover, this lens is also ideal for looking at whiteboards and documents at close range for example during meetings. When ordering an intermediate-distance bias pair of spectacles for the first time, we recommend that you actually try out a trial lens to ascertain “the range of vision this type allows and viewability” before proceeding with the purchase. Problems with viewability may gradually begin to manifest themselves if you do not first try out the lens before placing an order.
【4】Close-distance wide type ( For PC use)
Although this type of lens features less depth than the intermediate-distance bias type, it gives a wide field of close-range vision. This lens may be thought of as an intermediary type between the ordinary single vision lens for correction of presbyopia and the intermediate-distance lens. This type is ideal for people needing a wide field of close-range vision for desk work. For example, the ordinary single vision lens gives the wearer a field of vision of no more than about 30 cm ahead, and while this enables the wearer to see the keyboard when working on a computer, it may be difficult to see the monitor. The close-distance wide lens eliminates this problem, making it ideal for users working on computers.
2. Points in choosing Progressive Lens
“Progressive lenses” can be selected to match specific purposes, greatly enhancing usability. Be sure to select the ideal lens to match to your needs – e.g. driving, eating and drinking or working with computers.
When consulting with your optometrist on lens selection, try giving accurate information such as the main situations in which you will be using the spectacles, what kind of range and field of vision you require and aspects of your current spectacles with which you are dissatisfied. No doubt, he/she will give you sound advice that will help you to select the ideal lens for your needs.
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