OPTE key terms

The measure of how strongly light rays converge toward or diverge from a point, expressed in diopters as the reciprocal of the distance in meters. Positive vergence means converging light; negative means diverging.

The unit of optical power equal to the reciprocal of focal length measured in meters. A 2.00 D lens focuses parallel light at 50 cm; the unit also expresses vergence and refractive error.

The distance from a lens or mirror to the point where parallel incoming rays converge or from which they appear to diverge. It equals the reciprocal of power in diopters.

A blurring defect in which rays passing through the periphery of a spherical lens or the cornea focus at a different point than central rays. It worsens with large pupils and wide apertures.

An off-axis optical aberration in which oblique rays form a comet-shaped blur rather than a point. It increases with distance from the optical axis and with lens tilt or decentration.

A refractive condition in which power differs across meridians, so light from a point object does not form a single focal point. It may arise from the cornea, lens, or both.

A lens power that acts in only one meridian, correcting astigmatism. The axis specifies the meridian with no added power; the cylinder value acts 90 degrees away.

A single spherical power that approximates a spherocylindrical prescription, calculated as sphere plus half the cylinder. It is used for contact lens estimation and retinoscopy cross-checks.

The distance between the back surface of a spectacle lens and the front of the cornea, typically 12 to 14 mm. It matters when converting high-power prescriptions to contact lenses.

The power a lens actually exerts at the eye once its position changes relative to the cornea. Moving a strong lens closer or farther alters the vergence reaching the eye.

A calculation stating that induced prism in prism diopters equals lens power in diopters multiplied by decentration in centimeters. It predicts unwanted prism from off-center lenses.

A unit describing prism strength: one prism diopter displaces an image by 1 cm at a distance of 1 m. It quantifies strabismus deviation and prescribed prism.

The point on a lens through which the optical axis passes, where no prism is induced. Proper centration aligns it with the pupil to prevent unwanted prism.

The curvature of the front surface of a spectacle or contact lens, usually stated in diopters of radius. It strongly influences the fit and comfort of rigid contact lenses.

Measurement of corneal curvature, typically over the central 3 mm, yielding radius of curvature and corneal power. It guides contact lens base curve selection and astigmatism assessment.

An objective refraction technique in which a light streak is swept across the pupil and the movement of the reflex guides lens neutralization. It requires no patient response and is useful in children.

Paralysis of the ciliary muscle produced by agents such as cyclopentolate or atropine, temporarily removing accommodation. It allows accurate refraction in children and relaxes spasm.

Dilation of the pupil caused by agents such as tropicamide or phenylephrine, or by reduced light. It widens the view of the fundus but may trigger angle-closure in narrow angles.

The increase in crystalline lens power produced by ciliary muscle contraction, allowing focus on near objects. It declines with age, producing presbyopia.

The age-related loss of accommodative ability, typically becoming symptomatic in the mid-forties. It is managed with reading addition, multifocal lenses, or monovision.

The maximum increase in optical power the eye can achieve, measured in diopters. It is assessed by push-up or minus-lens techniques to determine reading additions.

The refractive state in which parallel light from infinity focuses on the retina without accommodation. The eye is optically neutral and needs no distance correction.

A refractive error in which parallel light focuses in front of the retina, blurring distant vision. It is corrected with minus lenses and is associated with axial elongation and retinal risk.

A refractive error in which parallel light would focus behind the retina when accommodation is relaxed. Young patients may compensate through accommodation, masking the error.

A condition in which the two eyes have unequal refractive power, commonly defined as a difference of at least 1.00 D. It can cause aniseikonia and amblyopia in children.

A perceived difference in the size or shape of images seen by the two eyes. It often follows anisometropic correction and is minimized by reducing power differences or using contact lenses.

Reduced best-corrected visual acuity in one or both eyes caused by abnormal visual experience during development, without structural cause. Treatment is most effective in early childhood.

A misalignment of the eyes in which the visual axes do not point at the same object. It may be constant or intermittent, and can lead to amblyopia and loss of binocularity.

A binocular assessment in which one eye is occluded while the other fixates, revealing manifest or latent deviation by the movement on uncovering or alternate covering.

The perception of depth derived from slight differences between the images received by the two eyes. It requires good visual acuity and alignment in both eyes.

Examination of the anterior chamber angle using a mirrored or prismatic contact lens. It distinguishes open from narrow or closed angles and identifies peripheral anterior synechiae.

Measurement of intraocular pressure, commonly by applanation or non-contact methods. It is central to glaucoma detection and monitoring, though pressure alone does not define the disease.

Enlargement of the central depression of the optic nerve head, reflecting loss of retinal ganglion cell axons. Progressive cupping with thinning of the neuroretinal rim suggests glaucoma.

A handheld grid test in which the patient reports distortion or missing areas in central vision. It screens for macular disease such as age-related macular degeneration.

The extent of space visible to a fixating eye, mapped by perimetry to detect scotomas and peripheral loss. Patterns of defect help localize retinal, optic nerve, or neurologic disease.

A patient's voluntary agreement to a procedure after receiving understandable information about its purpose, risks, benefits, and alternatives. It must be documented and may be withdrawn.

The level of care a reasonably competent practitioner in the same field would provide under similar circumstances. It is the benchmark used to judge clinical and ethical conduct.