With so many different lenses on the market today it’s no wonder that many people are asking “What are Bifocals?”
Bifocals help those that require vision correction for myopia (short-sightedness) and presbyopia (long-sightedness). The lenses allow clear vision at either distance within one pair of spectacles.
They can be thought of as ordinary prescription lenses with an additional reading segment. The reading segment is usually positioned towards the lower part of the lens.
Brief History Of Bifocal Lenses
It is thought that Benjamin Franklin invented Bifocal lenses in the
late 1760s. However, evidence now suggests that others may have worn
“Split Bifocals” before him. As he aged, Franklin required separate
glasses for distance and near use. Growing tired of constantly
changing his spectacles, he decided to do something about it. His
solution was to cut the lenses from each pair in half. He then split
the lens into two halves. Distance lenses at the top. Reading at the
bottom. Franklin could now use one pair of glasses for clear vision at
both distances. He originally named them “Double Spectacles”.
Modern technology has certainly helped to improve
Bifocals. They are now more durable and stylish. As well as more
widely available. But despite these technological advances, they still
work largely in the same way. As he aged, Franklin required separate
glasses for distance and near use. Growing tired of constantly
changing his spectacles, he decided to do something about it.
His solution was to cut the lenses from each pair in half. He then split the lens into two halves. Distance lenses at the top. Reading at the bottom. Franklin could now use one pair of glasses for clear vision at both distances. He originally named them “Double Spectacles”.
Modern technology has certainly helped to improve Bifocals. They are now more durable and stylish. As well as more widely available. But despite these technological advances, they still work largely in the same way.
Different Types Of Bifocal Lenses
Many variations of Bifocal have since been designed to suit various uses. Opticians are likely to recommend one of the following types of Bifocals.
D Seg Bifocals
The reading area of the D Seg Bifocal looks like a letter D positioned
on its side. They may be referred to as a Flat Top Bifocal. The D Seg is
the easiest Bifocal design for a wearer to adapt to. Because of this,
it’s the most popular type on the market today.
The reading
segment is available in various types. The most widely used lenses are D
28 and D 35. The D 28 segment is 28mm wide and the D 35 is 35mm wide.
Round Bifocals
As the name suggests, the round bifocal is round at the top. They were
originally designed to help wearers reach the reading area more easily.
However, this reduces the width of near vision available at the top of
the segment. Because of this, round bifocals are less popular than the D
Seg.
The reading segment is most commonly available in 28mm and 25mm sizes.
The R 28 is 28mm wide at the centre and the R25 is 25mm wide.
Executive Bifocals
The Executive Bifocal is similar to the original Split Bifocal worn by
Benjamin Franklin. The entire top portion of the lens is dedicated for
distance use whilst the bottom portion is for reading. They offer a much
wider reading area than other types of bifocal.
It’s not all good news, however. As mentioned earlier, the entire bottom
half of the lens is dedicated to reading. This prevents the edge profile
from being uniform, meaning the lenses are more difficult to glaze. This
can lead to lenses looking unsightly when mounted into frames. Those
with higher prescriptions and a taste for exotic styles or semi-rimless
frames will want to avoid executive bifocals.
Curve Top Bifocals
The Curve Top Bifocal, or the C Bifocal, is almost identical to the D Seg. A slight curve at the top of the lens is the only difference. Once more, the near vision segments are available in different widths. The C 28 is the most popular size available.
Double D Bifocals
Double D Bifocals are designed for those that require an intermediate or
reading segment at both the top and bottom of the frame.
At first, you may think, why on earth would you need near vision
available at the top of your glasses? However, there are some situations
where this would be very beneficial. Think of an electrician wiring a
light near a ceiling or a decorator cutting in around the coving. In
these cases, a near or intermediate segment at the top of the frame
would be very useful.
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