Eye terminology explained
Words your optometrist might use…
The terms here are the most commonly used by your optician, most will only be of use in special cases or with people with ‘complex’ prescriptions.
However, if you would like to know a little more about your eyes and the words used to describe their health please read on!
Any defect or a distortion in any optical system.
The process by which the eye increases its power to focus on a near object, by effectively changing the shape of the crystalline lens.
ACUITY, VISUAL (Often abbreviated to V.A.)
The capacity of the eye to clearly see the detail of an object at a specific distance.
A lens, or an optical system, of zero power.
A condition characterised by low V.A. that is not correctable by optical means.
A defect in the refractive state of the eye, e.g. myopia, astigmatism, etc.
This when there is a difference in the prescription of each eye, typically greater than 1.00D
A condition in which one eye is myopic and the other is hypermetropic.
The ocular condition in which the crystalline lens of the eye is absent, or has been
Any curved surface that is not spherical, e.g. elliptical
A condition of the eye where the principal meridians have different refractive errors, requiring a toric lens to fully correct the vision. (Often caused when the corneal surface is not spherical, being more like the surface of a barrel or a rugby ball).
For more information on astigmatism please click here.
Relating to both eyes.
An instrument designed for detailed examination of the eye, used particularly in contact lens practice. (Often referred to as a slit-lamp)
A chronic inflammation of the eyelid margins.
For more information on blepharitis please click here.
The angle formed by the upper and lower lids at the nasal and temporal ends.
Partial or complete loss of transparency of the crystalline lens substance or its
For more information on cataracts please click here.
A surface shaped like the inside of a sphere.
Movement of the eyes turning inwards, i.e. towards each other.
A surface shaped like the outside of a sphere.
The transparent anterior portion of the eye.
The displacement horizontally and/or vertically of the optical centre of a spectacle lens – away from the boxed centre – to coincide with the centre of the patient’s pupil.
The unit of measurement of refractive power of a lens.
(Double vision) A condition where objects are seen ‘double’.
The region of the retina, where the optic nerve joins the eyeball.
Defect of an optical system resulting in an image that does not correspond to the
shape of the object.
Movement of the eyes, turning away from each other.
Perfect sight, when light from an object at infinity is sharply focused on the retina.
The muscles attached to the outside of the eyeball that control the movement of the eyes. (There are six extrinsic muscles)
The act of directing the eyes to an object so that its image is formed on the most sensitive part of the retina.
An instrument that measures the power of a spectacle lens, or contact lens.
The back of the eye as viewed with an ophthalmoscope.
An eye disease characterised by increased intra-ocular fluid pressure.
For more information on glaucoma please click here.
Surgical procedure to replace damaged structures (e.g.cornea) with donor tissue.
A type of plastics material that contains water, commonly used in the manufacture of soft contact lenses.
This is long sight, when the eye is too short or its refractive power too weak and is corrected with a positive powered lens.
INJECTION (e.g. conjunctival )
Redness of the eye caused by the dilation (expanding) of superficial blood vessels in the sclera (white of the eye).
The horizontal distance measured between the pupil centres. Usually abbreviated to PD.
The coloured part of the eye surrounding the pupil
A condition causing inflammation of the iris.
For more information on iritis please click here.
An inflammation of the cornea of which there are many different types.
An instrument used to measure the curvature of the front surface of the cornea.
Relating to the tears.
Concentrated beam of light (Light Amplification by Stimulated Emission of Radiation) used in various types of surgery.
Eyeglasses for occasional use, held in front of the eyes by a handle.
A curved lens, having one spherical convex and one spherical concave surface.
Contraction of the pupil. (A drug that causes this is a miotic)
A single lens, with or without a frame, worn by holding between the brow and the
Relating to one eye (or lens)
A lens with multiple powers eg a trifocal or progressive lens.
Dilation (enlarging) of the pupil. (A drug that causes this is a mydriatic)
‘Short sight’, in simple terms the eye is too long or refractive power of the eye is too
strong. Corrected with minus powered lenses. (See previous module!)
The ability to read/carry out close work, usually at a distance of 33 – 40 cm.
The process of determining the power of an unknown lens, using trial lenses of known power.
A device placed in front of an eye to effectively block vision.
OCULUS DEXTER (Latin)
Often abbreviated to OD Meaning Right eye
OCULUS SINISTER (Latin)
Often abbreviated to OS Meaning Left eye
Condition of a tissue or structure that is not transparent.
A hand held instrument for viewing the eye internally, in particular the retina.
The point on a lens through which a ray of light will pass undeviated.
The rigid bony cavity in the skull that contains the eyeball, extrinsic muscles and
The study, diagnosis, and non-surgical treatment, of the anomalies of binocular vision.
An instrument for measuring the extent and characteristics of the visual field.
The ability of a material to allow the passage of a gas or fluid, as in gas permeable (rigid) contact lenses
A lens made of a material that changes colour as a result of exposure to UV light and heat.
Condition of the eye where the accommodative ability of the eye has declined to a point where reading spectacles are required.
For more information on presbyopia please click here.
A transparent optical medium with two plane surfaces inclined at an angle to one another, used to deviate light in the correction of muscle imbalance or problems of convergence, for example.
An artificial eye, or implant.
The aperture within the iris.
This is the distance between the centre of the the pupils in each eye (measured in millimeters). Important in positioning the lenses of the glasses correctly in relation to the pupils so as to obtain the optimum vision; especially important in glasses with high prescriptions.
Reaction of certain parts of the eye to a stimulus, e.g. the pupil reflex being the reaction of the pupil to a light stimulus.
The light sensitive layer of cells lining the posterior part of the eye, like the film in a
Hand held instrument used to obtain an objective assessment of a patient’s refractive condition (their prescription)
SALINE, (‘NORMAL SALINE’)
A sterile solution of sodium chloride (salt) in water
The tough white opaque fibrous outer coat of the eyeball
An area of partial or complete blindness, surrounded by normal visual field
Attachment to the front of a spectacle frame, passing over the ear.
SLIT – LAMP (SLIT-LAMP BIOMICROSCOPE)
An instrument used to examine the anterior parts of the eye under high magnification. Used extensively in contact lens practice.
A condition in which the eyes are not directed towards the same point, when the patient is fixating. Therefore the images are formed at different points in the eyes resulting in ‘double vision’.
The combined process of generating, smoothing and polishing of a spectacle lens surface to a given curvature.
An instrument used for measuring the fluid pressure inside the eyeball (the intra ocular pressure [IOP])
A case containing pairs of + and – lenses, both spherical and cylindrical, used in sight testing.
An adjustable spectacle frame with rims designed to hold several trial lenses, used in sight testing.
Patients’ vision without spectacles or contact lenses.
Another expression for monocular, i.e. pertaining to one eye.
The distance (in mm) from the apex of the cornea, to the posterior surface of the spectacle lens or trial lens.
The capacity of the eye for seeing distinctly the details of an object at a specific distance.
Clear, colourless fluid, similar to raw egg white in consistency, which fills the space in the eye between the crystalline lens and the retina.