Eye terms glossary

Accommodation: Adjustment of the focusing power of the eye to see objects clearly over a range of distances. Achieved by change in the shape of the crystalline lens. There is a reduction in focusing ability with age (presbyopia).

Astigmatism: A refractive error usually caused by toric (rugby ball-like shaped) curvature of the front surface of the eye. Instead of being brought to a focus at one point at the back of the eye, light is focused in two lines at right angles to each other. Astigmatism is easily corrected with glasses to bring the two lines to one point focus.

Bifocal lens: Spectacle lens having two portions with different focusing power. The top portion is usually larger and used for distance vision while the lower portion is smaller and used for near vision.

Blepharitis: Inflammation of the eyelids, due to infection, usually accompanied by crusts or scales on the eyelid margin. Eyes may become red, sore and itchy.

Find out more:
National Eye Institute blepharitis resource guide

Binocular Vision Anomalies: Eye problems affecting the way a pair of eyes work together.

Cataract: Loss of transparency ('clouding') of the crystalline lens of the eye. Mainly associated with age, but also with systemic conditions, trauma, exposure to UV light and Down's syndrome. Treatment, which involves surgical replacement of the cloudy lens, has a very high success rate.

Find out more:
National Eye Institute facts about cataract

Contrast Sensitivity: Most tests for contrast sensitivity measure ability to see low contrast letters/pictures. Reduced contrast sensitivity is usually associated with difficulty in conducting day-to-day visual tasks.

Dispensing Optician: A person qualified to advise on, choose, fit and modify spectacle frames and lenses.

Fundus: Name given to the structures at the back of the eye; includes the retina, optic disc, macular, and retinal and choroidal blood vessels.

Glaucoma: A group of eye conditions causing progressive vision loss. These often symptom-free conditions result in damage to the optic nerve at the back of the eye. The three main eye tests for glaucoma are eye pressure, optic disc appearance and visual fields.

Hypermetropia: Also known as 'long sight'. A refractive error of the eye in which light is brought to focus behind the retina (corrected using positive lenses). Uncorrected hypermetropia may cause problems with near work, for example headaches when reading. Significant, uncorrected hypermetropia in childhood may cause other more serious problems.

Intra Ocular Pressure (IOP): Pressure inside the eye, measured using a machine called a tonometer. A tonometer can measure IOP by direct contact with the front of the eye, or by using a 'puff of air'. Eye pressure that is very low or very high can cause eye damage, and referral to an ophthalmologist is required.

Keratoconus: Progressive eye disorder in which the cornea becomes irregular and conical. The condition usually effects both eyes, but one may be affected well before the other. The onset of the condition is often around puberty. Vision becomes affected when the coning leads to increased astigmatism. In the early stages glasses can be used to correct vision. In more advanced cases hard (Gas Permeable) contact lenses are the preferred treatment. In very advanced cases, surgery to replace the cornea with a graft is often successfully performed with good visual results.

Find out more:
National Keratoconus Foundation (USA)
Center for Keratoconus (USA)

Myopia: Also known as 'short sight'. A refractive error of the eye in which light is brought to a focus in front of the retina, causing distance vision to be blurred (corrected using negative lenses). Uncorrected myopia may cause problems with distance vision, for example not seeing road signs clearly when driving.

Nystagmus: Continual, regular, uncontrollable movement of the eyes affecting detail vision. The condition may vary and is often worse when a person is under stress or is tired; as a result vision may vary.

Find out more:
Nystagmus Network (UK) information page
Nystagmus Network (USA) information page

Ocular Motor Eye Anomalies: problems affecting the muscles which move the eyes (the extra-ocular muscles).

Ophthalmologist: Medical eye specialist usually based at a hospital eye department or clinic.

Optometrist: Also known as an 'ophthalmic optician'. Prescribes glasses and checks eye health.

Orthoptist: Checks vision and eye alignment, and treats eye turns (squints) and other binocular vision anomalies, usually in a hospital or clinic.

Squint / Strabismus: A constant or occasional eye turn, which may be seen in one eye or alternate between eyes. If the turn persists during childhood, then the affected eye may become 'lazy'. Treatment to improve vision in the 'lazy' eye involves patching the 'good' eye. Treatment is most effective if started before school age.

Find out more:
National Health Service Direct information about strabismus

Visual Acuity: The smallest, fine detail you can see. Usually measured with a letter chart. But, can also be measured using pictures, black and white lines or visual evoked potentials (VEPs).

Visual Evoked Potentials: Measure the response of the brain to visual stimulation. The easier the stimulus is to see, the larger the response measured.

Visual Fields: Vision in the periphery, or 'side' vision. A normal binocular visual field is about 190 degrees horizontally and 140 degrees vertically.

Visual Impairment: Reduced vision as a result of a disorder or disease of the eye or brain. A person can be registered 'blind' on the advice of an Ophthalmologist, if they cannot see the biggest letter on a standard (Snellen) letter chart with both eyes open when it is held 3m away.