Have you heard people using the terms Varifocal or Bifocal lenses and wondered what they are and how they work? This guide is aimed at helping you understand exactly that…
What are they?
Varifocal lenses, also called progressive addition lenses; progressive power lenses or graduated lenses, are corrective prescription lenses used in glasses to correct presbyopia and other disorders of accommodation.
How they work
A gradient of increasing lens power is added to the correction for the additional refraction error, ranging from a minimum correction, or nothing, at the top of the lens to maximum magnification at the bottom of the lens, the following is an example:
Distance vision is achieved through the upper part of the lens where there is a small amount of soft focus at the edges of your vision, or none, depending on your individual needs.
Arms length vision
Vision at arms length is through the middle part of the lens. There is more soft focus at the edges of your vision in this part of your lens.
Near vision is achieved through the lower part of the lens. There is some soft focus at the edges of your vision in this part of your lens.
A wearer can then adjust the lens power required for clear vision at different viewing distances by tilting his or her head to place the line of sight through different parts of the lens.
Varifocal lenses help to avoid the difficulties field of vision which can be created by bifocal lenses for example. The lenses are also more cosmetically attractive.
Issues with Varifocals
The lenses suffer the disadvantage of creating regions of aberration away from the optic axis, yielding poor visual resolution. Although manufacturers are constantly striving to minimize these distortions, some wearers find it difficult to cope with these visual anomalies and therefore cannot tolerate Varifocal lenses.
What are they?
Bifocal lenses are divided into two areas each of which has different optical powers. You are most likely to need to wear glasses with Bifocal powers if you have presbyopia, and an additional problem of myopia, hypermetropia and, in some cases, astigmatism.
Taking advantage of that fact that when a person views something at close range and that they usually look down and the reverse is true when they view at a distance, the early split 'Franklin' bifocals were designed with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Originally the lenses were simply cut in half and combined together in the rim of the frame.
Until recently, there were few choices for people who suffered from nearsightedness or farsightedness. You had to go for thickish lenses which had a visible line down the middle, add to that the limitations on the range of frames, which were mostly thick, large and far from attractive to accommodate the large lenses and your choice was to say the least, limited. A lot of people also faced adjustment problems with bifocals and feelings of dizziness while climbing up or down stairs. The adjustment process was difficult for some.
Where we are now…
In recent times examples involved the cementing of a bifocal segment onto a larger lens, but now most bifocals are made by moulding a small reading segment into the lens. Bifocal lenses are now available with the reading segments in a variety of shapes and widths. The most popular is the flat-top or D-shaped segment, 28 mm wide.