Lens types
Spectacle lenses come in three main types; single vision, bifocal and varifocal
All these lens types can be made with thinner and lighter materials and given improved performance with lens coatings
Single vision – a single vision lens is a lens with a single focal point i.e. Will correct distance vision if and you are longsighted and reading vision if you are longsighted
Bifocals – you eye has a flexible lens that can be focus by eye muscles so that you can see clearly up-close, typically by the mid-forties the lens hardens and muscles weaken so that near focusing becomes difficult. This is called presbyopia. A bifocal basically is distance lenses with a reading prescription added to the lower third. There is a clear demarcation or line between the distance and reading powers. This is why they are called ‘bi’focals i.e. two areas of focusMost people get used to using bifocals easily and they are far more convenient than carrying around a separate pair of glasses for distance and reading
Varifocals – Varifocals are similar in concept to bifocals except there is a gradual change from distance and reading power. This change is blended so there is no demarcating line. This allows focusing at all distances from very far away, to intermediate distances (e.g. computer screen) through to close up reading. This is why they are called ‘vari’focals.
They could be thought of as an advanced bifocal
Occupational lenses/lifestyle lenses – Lifestyle and occupational lenses are relatively new types of lenses designed to cope with the visual demands of modern life. For example, as more and more people are spending longer on their VDUs and tablets at home and work, the increasing visual demand is creating eyestrain and excessive tiredness that would not have been an issue in the past. Lenses can be prescribed to alleviate these symptoms for people who have otherwise normal eyesight such as the Anti-Fatigue or Varilux Computer lens from Essilor or the Hoyalux iD WorkStyle from Hoya
Lens materials
Plastic – This is the standard material used in spectacle making. It is an all rounder that is relatively tough, light and has good optical qualities
Glass – This has been superseded by plastic as it is heavier and can smash if dropped. However it has a very hard surface making it more scratch resistant than standard plastic
Polycarbonate – Polycarbonate is up to 10 times tougher than regular plastic and therefore very useful for children’s glasses, safety glasses and sports eyewear e.g. squash goggles. It also is used in rimless and semi-rimless due to its toughness. It also has the advantage of being thinner and lighter than regular plastic lenses
Hi-index – These are lens materials designed to minimize thickness and improve the cosmetic appearance of the glasses. They are described by their index number, the higher the number, the thinner the lens will be for your prescription. At the moment the thinnest plastic available is 1.74 index and thinnest glass is 1.9 index
Aspheric – These lenses are designed to minimise the amount of material in the lens keeping it thin and light. It works particularly well for (+) plus powers. (Long-sightedness)
Lens coatings
Anti-reflection – this coating virtually eliminates annoying reflections off the lens surface. This is useful when driving, particularly at night or if you use a VDU. Cosmetically, anti-reflection coatings improve appearance of your spectacles by making them look less ‘glassy’ and can make thick lenses look thinner
Scratch resistant – lenses can be toughened on the surface to make them more scratch resistant. This is particularly useful if you put your glasses on and off a lot or work e.g in construction or with children
