Lens designs

The main lens designs are single vision, bifocal, trifocal and progressive lenses. Single vision lenses have only one power in the lens and allow you to see clearly in either the distance or at a certain focal point at near. Most younger people who wear spectacles have single vision lenses but as people get older, they gradually lose the ability to alter the focus of their eyes ('presbyopia'), and generally between the ages of 40 and 50, they may 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 have more than one power, allowing the wearer to focus at different distances. Bifocal lenses have two distinct segments with different powers. Usually the top segment is used for distance viewing while the lower segment is used for reading. Trifocal lenses are prescribed for people who have a need to focus on the mid range in addition to near and distance. 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. Trifocals have been surpassed by the newer "progressive" lenses where there is a gradual shift in focal power from distance to near with no harsh dividing lines or “jump” between focal areas. 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. These lenses provide more natural vision than bifocals or trifocals and are aesthetically more attractive. In numerous studies the newer progressive lenses have been the preferred choice for wearers requiring a distance, intermediate and near focus option from their spectacles. There are more than 100 progressive lens designs on the market, the better designs having less peripheral distortion with wider intermediate and near focus regions within the lens. Some of the early designs people found difficult to adapt to and although many of these early inferior design lenses are still available (usually at discount optometry chains) nowadays they sit at the lower price end of the market. Without doubt progressive lenses have come a long way since they were first introduced over 50 years ago.

Lens materials

There are two basic types of spectacle lens materials: glass and ‘plastic’. The plastic lenses are then broken up into several types, each version having varying refractive index, optical, and strength and impact resistance qualities. Glass is the traditional lens material - it has excellent optical properties, is reasonably scratch-resistant and is relatively inexpensive. However, glass has two major disadvantages - it is heavier than plastic and polycarbonate, and it breaks easily. A shattered glass lens can cause horrific injuries to an eye. This makes them unsuitable for anyone who is exposed to potential impacts, including anyone at risk of falls or for playing sports (some may remember the infamous Bob Hawke verses cricket ball incident). Consequently, glass lenses have been largely superseded within the spectacle lenses industry.

The most common lens materials are hard resins, usually called ‘plastic’. Plastic lenses are lighter than glass lenses and are considerably more impact resistant but are more prone to scratching unless they have a scratch-resistant coating. Nowadays nearly all plastic lenses come with a scratch resistant coating that renders their scratch resistance similar to glass lenses. Currently there are no commercial spectacle lenses are completely scratch proof. 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 and lighter. 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 difficult to manufacture and thus more expensive than the standard index plastic and glass lenses.

Polycarbonate and Trivex lenses are highly impact resistant versions of ‘plastic’ lenses which make them the materials of choice for prescription safety glasses, sporting applications and rimless frame designs. Polycarbonate lenses are thin and light, although their optical quality may not be quite as good as that of Trivex lenses.

We can recommend the best lens material option for your individual needs.


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