Glossary of Terms

Refractive Error

The measure of the error of focus of an eye compared to an assumed normal point of zero. The error is measured in dioptres which is the reciprocal of the focal length in centimetres. The refractive error will include measurements for myopia (short sight), hyerpmetropia (long sight) astigmatism and presbyopia (the loss of focusing power due to age for near work).

Visual Field

The area of vision to the side when one is looking straight ahead. Defects in the sensitivity of the peripheral vision can be helpful in diagnosing many types of visual and general health conditions.

Common Eye Problems

Early detection of potential eye problems, by the optician, can prevent greater problems developing and is essential in their correct treatment.

  • Short sightedness (myopia) - means that the person can see things easier close up than far away. It is most commonly due to the eyeball being slightly longer than average.
  • Long sightedness (hyperopia) - means that the person has more difficulty seeing things close up than far away.
  • Astigmatism - means that the curvature of the front clear surface of the eye is curved more steeply on one meridian than another i.e. like a rugby ball.
  • Presbyopia (old sight) - means that the crystalline lens located within the eye becomes harder and heavier with age and loses its ability to focus on things close up.
  • Cataract - is a condition where the crystalline lens becomes opaque. Early symptoms are increased glare and a reduction in the ability to see at distance. Nowadays cataract surgery is very successful and a clear plastic insert is used to replace the opaque material in the lens.
  • Glaucoma - is a condition where the pressure inside the eye becomes raised, giving rise to pressure on the nerve fibres, which in turn can be destroyed causing a gradual reduction initially in the peripheral visual field. One eye is usually affected before the other and quite a lot of visual loss can occur before it is detected. It is important to have an eye test regularly, especially if you are over 40.
  • Macular Degeneration (AMD) - This is a condition which usually affects older adults and results in a loss of vision in the centre of the visual field. Recognising faces or reading becomes very difficult.