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.
Glasses lenses can be treated with anti-reflection 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 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 buy 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.
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.
As we enter our forties, many of us find that we need reading glasses for viewing things that are close to, for example packets, menus and our phone screens. This is presbyopia – a natural consequence of ageing.
‘Readymade’ reading glasses are available in many shops. These are handy as spare glasses but are only exactly right for you if you have the same prescription in each eye, and have no astigmatism. Many readymade spectacles are not made to the same standards as prescription glasses. They will not do you any harm, and are fine as spare/extra pairs, but we would recommend that you have a pair of glasses which are made specifically for you, especially if you do a lot of close work such as sewing. These will be made to your exact prescription, and fitted so that you look through the centre of the lenses, so you get the most clear, comfortable vision. They needn’t be expensive. Your optometrist can provide further advice.
If you choose readymade spectacles, it is still important to have regular eye examinations. As you get older, you are more likely to develop eye conditions, such as glaucoma, which does not cause any symptoms in the early stages. Treatment is more effective if conditions are caught early, so we recommend you have an eye examination at least every two years after you turn 40.
Take a look at our video to learn how to clean your glasses effectively without damaging the lenses.