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Glossary of eyesight terms

Accommodation: the eye's process of adjusting its focus from far to near.
Acuity: the fineness of detail the eye can distinguish, see resolution
AMD: Age related Macular Degeneration: see macular degeneration
Astigmatism: a focusing distortion which is present at all distances, it can be cancelled out by a cylindrical component in an ophthalmic prescription (marked CYL on prescriptions). The angle of slant of the distortion is called its axis.
Autorefractor: an instrument used by opticians to give a rough estimate of the eyes' focusing errors
Axis: see Astigmatism
Bates Method: a system of eye exercises which aims to reduce myopia by altering the tone of muscles associated with focusing the eye.
Bi-Focal: a spectacle lens which has one section for distance and one for near work.
Blur, Blur circle: when an object is out of focus each point of the object is spread out in a circle. The image is blurred.
Cataract: a clouding of the lens or cornea of the eye which obscures vision.
Ciliary Muscle: a ring of muscle inside the eye which adjusts the shape of a flexible lens to focus the eye at different distances.
Contact Lens: a dished transparent disc that rests against the cornea to alter its focusing power
Convergence: the amount the eyes rotate towards each other to point at a near object.
Cornea: the transparent domed window at the front of the eye.
Corneal Transplant: an operation to replace a cornea that isdamaged or diseased
CYL,Cylinder: see astigmatism
Diopter: the measure of lens strength. =1/(focal length in metres)
Emmetropia: the state of normal vision
Emmetropisation: the process of an eye changing shape towards normal vision
Eye Chart: a chart displaying letters or symbols in a range of sizes used to determine focusing ability. (See Snellen) Eye Exercise: various exercise systems for improving focusing or convergence
Eye Strain: aching muscles around the eyes usually resulting from the brain struggling to interpret blurred or dissimilar images from the eyes.
Focus: to bring light rays from an object together to form an image
Glaucoma: increased pressure of the fluid inside the eye.
Hypermetropia: see hyperopia
Hyperopia: the eyes cannot focus on near objects but see better in the distance
Iris: surrounds the pupil of the eye. It expands and contracts to change the size of the pupil and control the amount of light entering the eye.
Keratoconus: a mis-shaped cornea which cannot focus clearly. Sometimes a contact lens will sometimes help. Keratometry: measuring the curvature of the cornea
Lazy Eye: when the brain favours one eye the other may become weaker - a lazy eye
Lens: a transparent disk with curved surfaces used focus light
Lens Implant: an artificial lens used to surgically replace a diseased lens in the eye
Long Sight: see hyperopia
Macular: the central part of the retina where detailed focusing takes place
Macular degeneration: a condition which destroys the macular causing the loss of central, and detailed vision Multifocal: a spectacle lens which has several sections for focusing at different distances.
Myopia: the eyes can focus very near but cannot focus in the distance.
Myopic: decribes an eye or person with myopia
Pinhole: a small hole which can be looked through giving the eye an artificially small pupil.
Pinhole Glasses: spectacle frames containing opaque screens containing several pinholes. Used to relax or exercise the eyes.
Pinhole Test: a visual test using a pinhole to establish whether poor vision is due to poor focussing or fue to physiological or neurological factors.
Presbyopia: the loss of focusing flexibility of the eye with age making it more difficult to focus close to.
Pupil: the variable hole in the middle of the iris which admist light into the eye.
Resolution: a measure of the ability of the eye to distinguish fine detail.
Retina: the light sensitive lining of the rear part of the eye. it coverts the light image falling on it into nerv signals to the brain.
Retinoscopy: the technique of looking into the eye with an optical instrument to observe the retina.
Short Sight: see myopia
Snellen: a system for measuring the accuity of focusing using eye charts. The result is expressed as a fraction eg 20/20
Varifocal: a spectacle lens which has graduated focusing strength from top to bottom

©Peter Duthie 2000