The Eye and how it works

The eye is a spherical structure with a transparent window at the front the cornea. Behind this is the coloured iris with a central aperture the pupil which allows light to enter the eye.

The lens behind the pupil is a transparent, flexible structure that can adjust its shape and power to bring objects into focus on the retina at the back of the eye. At the centre of the retina is a small area called the macula that is highly specialised and enables you to see fine detail. Visual information is carried from the eye to the brain via the optic nerve.

Conditions which effect the performance of the eye

Myopia, or short sight, occurs when, for various reasons, the eye cannot focus distant objects clearly on the retina, but there is no problem when viewing near objects. The remedy for this is the use of a negative or diverging, lens of the appropriate power.

Hypermetropia, or long sight, is when the opposite occurs. The eye can see well for distance, but cannot focus near objects correctly. This is remedied by using a positive, or converging, lens of the appropriate power.

Astigmatism occurs when the eyeball is not symmetrical in shape it is flatter in one direction than the other. The eye ball is often described as being more rugby ball shaped than football shaped. Astigmatism can occur on its own or frequently in conjunction with myopia or hypermetropia.

As we get older, from about the age of 40, the lens within the eye becomes less flexible and gradually loses its ability to focus on nearer objects. This means that we start to experience difficulty with close work and have to hold our reading material further and further away. There are various ways of dealing with this problem:

Reading glasses – a pair of lenses which are specifically made for close work. These may be required in addition to any other spectacles that you are already wearing. Reading glasses are purely for viewing near objects, so any distant object viewed through them will be blurred.

Bifocals – these are lenses which have two distinct areas for viewing objects at different distances. The top portion of the lens is for long distance viewing whilst the bottom portion of the lens is used for close work. There is a visible dividing line separating the two portions of the lens, and the reading segments are available in a variety of shapes and sizes.

Progressive or Varifocal – these lenses incorporate viewing areas for all distances, without any visible dividing line. The power of the lenses gradually changes from top to bottom so that the top portion of the lens is used for long distance viewing, the lower portion for reading, and the middle area of the lens can be used for viewing intermediate distances, such as computer screens. There is, therefore, an area of the lens suitable for every viewing distance. The different areas of the lens are quite distinct and vision through the periphery of the lens can be compromised, therefore, when wearing progressive lenses for the first time, the eyes may take a little time to adjust to them.

There are many different types of bifocals and progressive lenses available, and the type most suitable for you will depend on your prescription and visual requirements.

The dispensing optician will be able to discuss your needs with you and help you select the type of lens that is best for you.

Other lens options

High index lenses – the majority of prescriptions and lens types can be made in materials that are thinner and lighter than traditional glass or plastic lenses. Such lenses can help make your spectacles more comfortable to wear and can improve their cosmetic appearance.

Anti-reflection coatings – when light rays meet a lens only a proportion actually pass through the lens the rest are reflected from the surface. For spectacle wearers the reflections occurring on both sides of the lens can be annoying and distracting. Reflections can impede the performance and appearance of the lenses, particularly under certain circumstances, such as driving at night and working under artificial lighting. An anti-reflection coating can dramatically reduce the amount of light reflected by a spectacle lens, thereby improving visual clarity and the cosmetic appearance of the lens.

Transitions lenses – these are plastic material lenses that change colour by reacting to different light levels. The lenses darken in sunlight and fade when indoors or in the shade. When in bright sunlight the lenses reduce the amount of light transmitted by up to 70 per cent and, therefore, protect you from glare and UV radiation. Transitions lenses are made in several colours and are available for most prescriptions and lens types.

There are obviously many factors to be taken into consideration when selecting your new spectacle lenses. We all have different lifestyles, visual needs and expectations prescription, work, sport and hobbies all play a part in helping us decide which lens is best more than one pair may be required. Whatever your needs, the optometrist and the dispensing optician will be happy to discuss the options and help you select the right spectacles for you.