Choosing the right lenses
People who wear glasses are prescribed either single vision lenses or multifocal lenses, and this distinction is made depending on the type of vision problem they are trying to correct. As the name suggest, single vision lenses allow the wearer to improve their focus in only one specified “distance” (such as for focusing on objects that are close to them or for focusing on objects in the distance). Multifocal lenses, on the other hand are designed for people who need help focusing in more than one “distance”. Multifocal lenses are divided into three main types: Bifocal, trifocal and progressive.
Single vision lenses
The most commonly prescribed lenses are ‘single vision’ lenses. Single vision lenses have only one power in the lens and allow you to see clearly in either the distance or close up. Most younger people who wear glasses have single vision lenses, but as people get older, they gradually lose the ability to alter the focus of their eyes (‘presbyopia’), and usually some time between the ages of 40 and 50, they develop a need for corrective lenses for both distance and near vision. They can use two pairs of single vision spectacles and change them as necessary or, alternatively, one pair of multifocal lenses. As the name suggests multifocal lenses are lenses which have more than one power, allowing the wearer to focus at more than one distance.
Bifocal lenses have two distinct segments with different powers. In most cases a top segment is used to see in the distance while a lower segment is used for reading and other close tasks such as sewing.
Trifocal lenses are prescribed for people who have a need to focus on the mid range in addition to things that are close up and at a distance. For example, bank tellers may need to read forms passed to them, see the customer standing in front of them and be able to see across the bank. Trifocal lenses have three segments; the top segment for seeing in the distance, a middle segment for seeing at intermediate distances and a lower segment for reading and other close work. There is a distinct dividing line between the segments.
Another way of providing multiple corrections in one lens is to have a gradual change between the distance and near corrections. This type of lens is known as a progressive addition lens (also known as a “PAL” or “progressive” lens). This type of lens allows the wearer to focus on all distances, there being a gentle change from distance to intermediate to near focus as one looks through different parts of the lens.
Progressive addition lenses provide more natural vision than bifocals or trifocals and are aesthetically more attractive. They are popular with people who think that the dividing lines in bifocal and trifocal spectacles make them “look old”.
Choosing the right lens material
Lenses are made out of three main material types: Glass, resin (plastic), and polycarbonate. Glass is the traditional lens material – it has excellent optical properties, is reasonably scratch-resistant and is inexpensive. However, glass has two disadvantages – it is heavier than plastic and polycarbonate, and it breaks easily. This makes it unsuitable for anyone who is exposed to potential impacts, or for playing sports.
The most common lens materials are hard resins, usually called ‘plastic’ or sometimes ‘CR39’ (CR39 is the trade name of the first resin widely used for making lenses). Plastic lenses are lighter than glass lenses and are considerably more impact resistant but are usually slightly thicker, and are more prone to scratching unless they have a scratch-resistant coating. There is little difference in the cost of plastic and glass lenses with the same prescription.
Both glass and plastic lenses are available with different refractive indexes. The refractive index of a material is a measure of the speed of light passing through that material, which influences how much a lens made of that material bends the light. For a given prescription, a higher refractive index will allow the lens to be thinner. High refractive index materials are particularly useful for people with high prescriptions, as they help to avoid the ‘coke bottle’ appearance of thick lenses. High refractive index lenses are more expensive than the usual plastic and glass lenses.
Polycarbonate lenses are highly impact resistant, which makes polycarbonate the material of choice for safety glasses and for sporting applications. Polycarbonate lenses are thin and light, although their optical quality may not be quite as good as that of glass or resin lenses, particularly in high powered lenses.
Why do some lenses have tints or coatings on them?
Tints are generally used for sun protection, although they can also be used simply for fashion purposes. Tints can be of almost any colour but grey tints are generally the best for preventing glare, as they do not distort colour perception. Colour perception is particularly important when driving, for recognising traffic signals.
Lenses can also be coated to reduce scratching. The coatings will help extend the life of a pair of glasses, although no coating can prevent all scratches. If you purchase plastic lenses, make sure that they have a scratch resistant coating.
Anti-reflection (AR) coatings reduce reflections from the lens surfaces, increasing the amount of light that passes through the lens. The extra light improves vision, particularly for older people. The reduced amount of reflection enhances the appearance of the glasses, and is particularly effective in improving the appearance of higher powered lenses and high refractive index materials. AR coatings are particularly useful when driving at night, when using computers, and when working under fluorescent lights.
What are photochromatic lenses?
Photochromatic lenses have a tint that automatically adjusts for the amount of light present. This is achieved by incorporating a chemical which becomes darker when it absorbs ultra-violet light. The more sunlight, the darker the lenses become. Photochromatic lenses allow one pair of spectacles to function as prescription sunglasses, as well as regular spectacles.
Photochromatic lenses are available in a range of tints, and in both glass and resin materials. They generally take a few cycles of darkening and fading to reach their full efficiency, so that after some use they become darker and change more quickly than when they are brand new. Photochromatic lenses are temperature sensitive, although this is generally noticeable only if they are left on the dashboard of a car in summer, for example, and photochromatic resin lenses may also fade after a few years.
Why do some lenses cost more than others?
The cost of optical lenses can vary, depending on the exact type of lenses. The cost depends on many factors, such as the lens design, the lens material, the use of coatings and tints, and the prescription. Generally speaking, more complex lens designs are more expensive, so that progressive lenses cost more than bifocals, which in turn cost more than single vision lenses. High refractive index materials are more expensive than materials with lower refractive indices, so that for a given prescription, thinner lenses will cost more than thicker lenses. Special features such as tints or anti-reflective coatings will add to the cost of the lenses. Similarly, lenses to correct high or complex prescriptions have to be custom ground and so cost more.