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Varifocal
Although Varifocal lenses are still considered by many people to be new
technology, they have been available for some 50 years. In this time the
design of the Varifocal lens has been refined and improved by the worlds
leading lens designers, but the basic principle remains the same.
The distance prescription is at the top of the lens. It is positioned
directly in front of the pupil so that when the wearer looks across the
room with a natural head position he or she can see clearly at a
distance.
The power of the lens then progressively becomes more positive as the
eye travels down the lens. Towards the bottom of the lens is the near
vision prescription (which is normally set to focus at around 14-16") to
enable clear vision for reading.
As the power of the lens has increased gradually, each point between the
distance and reading portions of the lens has a different focal point.
By selecting the
right part of the lens, the wearer can focus clearly at any given
distance. It is very important however that the right part of the lens
is used for the correct viewing distance. For example, when walking, if
the wearer glances down at their feet, the eye will naturally drop into
the reading zone of the lens (which will focus at around 16") and
therefore the wearers feet will be blurred. If the wearer drops their
head angle slightly, the eye will be in the intermediate power zone and
the feet will become clear again.
The
principle of head angle is of vital importance when using a VDU. Because
the intermediate power zone on the lens is just below eye level it is
very important that the VDU be positioned so that the wearer naturally
glances down at an angle of around 10 - 15 degrees. Many VDU users
position the screen so that it is on eye level. Positioned like this,
the wearer would be using the distance prescription zone on the lens and
therefore would have to raise their head slightly to see the screen
clearly.
Lens Technology
As we
mentioned earlier, Varifocal lenses have improved vastly over the
decades. But what does this actually mean?
To achieve a variation in power across the surface of the lens, there
are certain areas of distortion created. These are principally at the
periphery of the near and intermediate zones of the lens. Although the
lens designers are always striving towards a lens with no distortion, at
present no such lens exists.
These
distortions have a restricting effect on the width of the near and
intermediate zones of the Varifocal lens.
However,
with the advent of computer controlled lens design, we can now offer a
range of lenses with negligible amounts of distortion. Bespoke lenses,
designed around not only your prescription and eye position within the
frame, but also the position in which the frame is to be worn,
practically eliminate distortion and give a field of view never before
seen with a Varifocal lens. |
