Types of Glasses Lenses Explained
Recently we have had several occasions where a customer needs something other than standard single vision lenses in their TEYES frames. Most of the time it is the strength of the prescription that triggers the need for high index or aspheric lenses. There are also other conditions that require different lenses and I hope you find the following informative and useful. If you would like any more information about lenses don’t hesitate to contact us!
Single-vision lenses
Single vision lenses are the simplest form of lens. Concave lenses are used to correct short sight and convex lenses to correct long sight. Concave lenses
are thinner in the centre than they are at the edge and convex lenses are thinner
at the edge than in the centre. The curvature of the lens, its thickness
and weight will depend on how short- or long-sighted you are, the material it is
made from, the size and shape of the frame, and the distance between your
pupils. Most lenses are made of lightweight plastic and there is a wide
range of materials available to suit your prescription and lifestyle.
Bifocals and trifocals
All of us will develop presbyopia as we get older and will find it increasingly
difficult to focus on objects that are close to us. Reading glasses will help
but if you already wear glasses, it can be inconvenient to have to swap pairs in
order to see things at different distances. One solution is to wear
bifocal lenses, which give you clear distance and near vision in one pair of
glasses. The top of the lens corrects distance vision and the lower part
corrects near vision. As you tend to look down to read, you automatically look
through the correct part of the lens. There is a distinct dividing line between
the two parts. Trifocal lenses have three sections – one for distance, one for reading, and a middle part for intermediate vision. They have two dividing lines in the lens.
Varifocal or progressive lenses
Unlike bifocal or trifocal lenses, varifocal lenses have no visible dividing lines
between the different sections of the lens. The power of the lens changes
smoothly from your distance to your near prescription, allowing you to see
clearly at all distances.
High-index and aspheric lenses
High-index lenses are denser than conventional refractive index lenses,
and so lenses made of these materials are thinner than those made of
conventional materials. This is particularly important if you have a
strong prescription. Aspheric lenses reduce the distortion from the edges of
the lenses, and are also thinner and lighter than non-aspheric designs.
Scratch-resistant/hard coating
Plastic lenses are lighter than traditional glass lenses but they scratch more
easily, reducing the quality of your vision. Scratch-resistant coatings
protect against damage and prolong lens life, although they are not scratch
proof.
Anti-reflection coating
Glasses lenses can be treated with antireflection coatings to eliminate
distracting reflections from the lens surfaces. This is particularly helpful for
computer users and for night driving. Anti-reflection coatings also improve the
cosmetic appearance of your glasses and can help disguise the thickness of
your lenses.
Photochromic lenses
Photochromic (also known as sun-sensitive) lenses automatically darken when exposed to specific types of light, most commonly ultraviolet radiation. The brighter the sun, the darker the lenses become. They become clear again once out of the
sunlight. Photochromic lenses can be made of glass, polycarbonate, or another plastic. The performance of some of the older photochromic materials degrades over
time. If you notice that your photochromic lenses do not go as clear as they used to you should consider buying a new pair. When driving you may find your photochromic lenses do not become as dark as they do outside. Car
windscreens (and windows) filter and absorb most of the short wavelength
light (near UV) that would normally trigger the darkening process.
(Information provided by The College of Optometrists)
