More about Progressive Lenses

-




These lenses have several names, such as Progressive Lenses, Multi-Focal or Progressive Addition Lenses (PAL), but essentially they are three different lens prescriptions combined into one physical lens. The benefit of this is that only one pair of glasses is needed for driving, computer work and reading.
Progressive Lenses are often recommended when a person reaches the point where simple reading glasses are no longer enough because his/her distance vision has also deteriorated. This usually happens when a reading glass prescription is more than + 2.00 diopters and a person‘s visual system has adapted to the stronger glasses by the dimming of his/her distance vision.
This is when bi-focal lenses are needed. Bi-focals have one lens prescribed for distance, usually lower power, and another, usually of higher power, for reading. Such glasses were invented by Benjamin Franklin in 1784 as he was being afflicted by presbyopia and having trouble seeing both up-close and at a distance. Becoming tired of switching between two types of glasses, he devised a way to have both types of lenses fitted into one frame. The distance lens was placed at the top and the up-close lens was placed at the bottom.

Progressive Lens designs are variations of the hour-glass figure where the top part is the prescription for distance vision, the narrowest and centre portion for middle distance computer work, and the bottom part a more powerful prescription for reading. There are many different manufacturers and designs each with their own unique features.

However, all Progressive Lenses limit your visual field. You only have clear vision directly in front of you. Wherever you point your nose will be clear but the moment you want to use your peripheral vision, prismic distortions will appear. This distortion in viewing is often refereed to as the “swing effect,” the perception that objects are moving unnaturally in the visual field. Depending on how well the lenses are fitted there might also be limits to your vision in the vertical.

When using a Progressive Lens it is best to point your nose at what you want to look. Some say you have to turn your whole body towards what you are looking at for a clear view. If you find that the reading or computer zone is too small, you may need a pair of special reading or computer glasses where the entire lens is focused on the screen or up-close. This rather eliminates the benefits of Progressive Lenses. 

What are the problems?

  • Very often, problems are due to improper prescription and angle of view. The Progressive Lenses must be positioned very accurately otherwise you have problems using them. Even if the position of the lenses is only a few millimetres off you will struggle to get used to them. In fact, some people never get used to wearing Progressive Lenses at all so it is important to make sure that the vendor agrees to their return, usually within 30 days, before you pay for anything. Also, in some cases the lenses have to be remade.
  • Progressive Lenses are much more expensive than regular reading glasses. Optometrists say it is better to start wearing them early so you can become used to them, and can wear them when the lens power is stronger. Some may say that this is a way to make you dependent on them so that you will have to pay the high price.
  • All progressive and multi focal lenses limit your visual field. When you turn your eyes to the side your vision will be distorted due to the hour glass pattern in the lens design. Some more expensive lenses have a wider field but in the end all of them have this problem.
  • Your head position needs to be at a very specific angle for you to have good vision. If the lens band is not positioned correctly you will need to bend your head as well and keep your head in a fixed position for the duration of your work. This may lead to neck and shoulder pain.
  • When the lenses’ power goes up, your visual system adapts to them and your natural vision is diminished and, eventually, you will not be able to see anything without them. You will then need to live within the limitations of these lenses.
  • Due to the three different lens power bands that are incorporated in the Progressive Lens design you will experience a distortion of your perception of where the ground is. Depending on which band you are looking through, your sense of where your feet are in relation to the ground is altered. This is something some people cannot get used to. It is also the cause of falls, especially when walking downstairs.
World Health Organisation statistics show that falls are the second leading cause of unintentional injury related deaths worldwide. Progressive Lenses have large areas of prismatic displacement. Older people appear to adapt to the false projection of Progressive Lenses in the central visual axis but this adaptation has affected the visual-spatial stored information. This may lead to a fall, especially in unfamiliar surroundings.
Research shows that wearers of Progressive Lenses performed significantly worse in distance depth perception and distance edge contrast sensitivity tests in situations where they were forced to look through the lower segments of their glasses. The attributable risks for the Progressive Lens wearer were 32.2% for all falls, 40.9% for falls due to tripping, and 40.9% for falls outside the home. (Lord, S.R., Dayhew, J., Howland, A. J., Am. Geriatr. Soc., 2002 Nov; 50(11):17606).

Leo Angart



 

Comments

Post a Comment

Popular posts from this blog

Principles of Vision Training

-
Image
Principles of Vision Training When it comes to vision training, not all optometrists seems to be on the same page about it. It is admittedly baffling and for many, it is an inference that could make them go out of business. Hundreds of near sighted patients including children are restoring their natural eyesight and omitting their dependence on glasses totally.   Near sightedness or Myopia is an incredibly common medical problem that causes the patient difficulty in seeing objects at a distance. Data shows that the incidence of Myopia has increased in the past few decades. In the 1970s, only 25% of the population had Myopia. But now, about 42% myopics exist.   Myopia is a leading vision problem Myopia can be genetically determined or environmentally determined. Heredity plays an essential role. If the parents have myopia, then the children are at a high risk of inheriting the condition as well.   Additionally, children who spend long durations of time eng...
Read more

More about fluorescent lights

-
Image
Fluorescent light tubes was introduced by General Electric at both the New York and San Francisco World Fairs in 1932. Compact Fluorescent Light CFL was also developed by General Electric in 1973, but the 25 million dollars needed the build a factory was considered too expensive at the time so it was not produced outside the laboratory. The design was eventually copied by others. In 1995, helical CFLs, were manufactured in China and became commercially available. Since that time, their sales have steadily increased.   In 2005 the EU began phasing out incandescent light bulbs in favour of CFL’s the US followed in 2014. The rationale is energy sawing since the CFL are using less energy than the old-fashioned Edison incandescent lamp. However, the newer solid state Light Emitting Diodes L.E.D. technology is far more energy efficient. L.E.D lights use 80% less energy and has a lifespan approaching 40 years of continuous use and has no mercury. Naturalness of fl...
Read more

More about Astigmatism

-
Image
Eye doctors generally believe that astigmatism is mostly inherited and perhaps partly environmentally caused. They also believe that it does not go away and will probably get worse over time. While the aim of the vision-training astigmatism exercise is to eliminate corneal astigmatism by releasing the tension, it is useful to understand the way astigmatism is corrected with glasses, contact lenses or surgery. First some historical background. Sir Isaac Newton, who himself appears to have been astigmatic, first considered this question of astigmatism in 1727. Seventy years later, in 1801, the renowned scientist Thomas Young investigated astigmatism in detail. Young is reported to have had 1.7 diopters astigmatism. It was Cambridge astronomer Airy (1827) who was the first to correct the astigmatism with a cylindrical lens. However, it was the invention of the keratometer by Helmholtz in 1856 and the treaties of Donders in 1864, “Astigmatism and Cylindrical Lenses,” that put astig...
Read more