DHA Optometrist key terms

The refractive state in which light from a distant object focuses exactly on the retina with the eye's accommodation relaxed, so vision is clear without correction.

Any refractive error in which distant light does not focus on the retina when accommodation is relaxed, including myopia, hyperopia and astigmatism.

The increase in the eye's optical power produced by contraction of the ciliary muscle, which lets the crystalline lens thicken so near objects stay in focus.

The maximum amount of focusing power the eye can add, measured as the difference in dioptric power between the near point and the far point of clear vision.

The age-related loss of accommodative ability caused by increasing stiffness of the crystalline lens, producing difficulty with near tasks that usually becomes noticeable in the fifth decade.

A single value summarising a spherocylindrical prescription, calculated as the sphere power plus half of the cylinder power, often used for screening and comparison.

The meridian of a toric lens or astigmatic eye that has no refractive power in the correcting cylinder, expressed in degrees from 1 to 180.

A form of astigmatism in which the steepest corneal meridian is vertical or near vertical, so the vertical meridian has greater refractive power than the horizontal.

The distance between the back surface of a spectacle lens and the front of the cornea, which must be accounted for when converting prescriptions above about four dioptres to contact lens power.

The refractive power of a lens measured from its posterior surface, which is the value used when specifying spectacle prescriptions and converting to contact lenses.

The distance between the examiner and the patient during streak retinoscopy, whose reciprocal in dioptres must be subtracted from the neutralising lens power to obtain the true refractive error.

The point in retinoscopy at which the reflected light fills the pupil and reverses direction, indicating that the lens in the trial frame has neutralised the patient's refractive error at that meridian.

Refraction performed after paralysing the ciliary muscle with a drug such as cyclopentolate or atropine, removing accommodative influence and revealing the full hyperopic or latent error.

Enlargement of the pupil, produced either pharmacologically by parasympatholytic or sympathomimetic drops or pathologically by neurological or traumatic causes.

A drug that constricts the pupil by stimulating the parasympathetic supply to the iris sphincter, examples being pilocarpine and other cholinergic agents.

The fluid pressure inside the eye, normally maintained between roughly 10 and 21 millimetres of mercury by the balance of aqueous humour production and outflow.

Examination of the anterior chamber angle using a mirrored contact lens or prism, allowing assessment of angle width and detection of angle closure or abnormal vessels.

Magnified examination of the anterior and posterior segments using a controlled slit of light and a stereoscopic microscope, forming the core clinical instrument of ophthalmic practice.

Visual examination of the interior of the eye, particularly the retina, optic disc and macula, using an ophthalmoscope or a slit lamp with a condensing lens.

The proportion of the optic disc occupied by the central pale cup, a key measure in glaucoma assessment whose enlargement or asymmetry may indicate optic nerve damage.

Measurement of the curvature of the anterior corneal surface, usually expressed as radii in millimetres and powers in dioptres along the two principal meridians.

A progressive non-inflammatory condition in which the cornea thins and steepens into a cone shape, causing irregular astigmatism and reduced best-corrected vision.

A contact lens made of rigid oxygen-permeable polymer that maintains its own shape on the eye, offering clear vision for irregular corneas and long-term durability.

The curvature of the back surface of a contact lens, chosen to align with the corneal curvature so the lens centres well and fits comfortably.

Refraction performed while a contact lens is worn, used to verify that the lens power produces good vision and to refine the final prescription.

The additional positive power incorporated in the near portion of a multifocal spectacle or contact lens prescription to compensate for reduced accommodation.

A spectacle lens whose power increases smoothly from the distance zone to the near zone, providing continuous focus at intermediate distances without a visible dividing line.

An optical element that deviates light toward its base, used in spectacles to relieve diplopia or to measure and manage ocular misalignment.

A latent tendency for the eyes to deviate when fusion is disrupted, held in alignment during normal viewing by fusional vergence and revealed by dissociation tests.

A dissociation test in which one eye is covered while the other fixates, allowing observation of any movement that reveals a manifest or latent deviation.

The closest point at which the two eyes can maintain single binocular vision while converging, measured by bringing a target toward the patient along the midline.

Reduced vision in one or both eyes caused by abnormal visual experience during development, not correctable by lenses and not explained by structural eye disease.

A manifest misalignment of the eyes in which the visual axes do not point at the same object, classified by direction as esotropia, exotropia, hypertropia or hypotropia.

Treatment of amblyopia by patching or otherwise blurring the better eye so the weaker eye is forced to take up visual tasks and develop improved acuity.

The sharpness of central vision, measured by the smallest letters or symbols a patient can identify at a standard distance and recorded as a fraction or logMAR value.

The ability to detect differences in luminance between an object and its background, measured across spatial frequencies and often reduced even when standard acuity is normal.

An optical or electronic device such as a magnifier, telescope or filter that helps a person with irreversible vision loss perform daily tasks more effectively.

A patient's voluntary agreement to an examination or treatment after receiving an explanation of its purpose, risks, alternatives and consequences, documented in the clinical record.

The contemporaneous written or electronic documentation of a patient's history, findings, diagnosis and management, which must be accurate, legible and retained as required by regulation.

The formal transfer of a patient's care to another practitioner or specialty when findings exceed the optometrist's scope or require medical or surgical management.