Eye Care Glossary


A characteristic of a lens that prevents the formation of a perfect image. Aberrations affecting the quality of images produce degraded sharpness, lowered contrast, distorted shape, and color fringing.


Technique achieved with excimer laser in which tissue is removed from the central optical zone with the intent of reshaping the cornea’s curvature; usually for correcting nearsightedness.


The ability of the eye to change its focus from distant object to objects closer than optical infinity. The eye achieves this by altering the shape of the crystalline lens with the ciliary muscles.


Clearness, as in visual acuity. The most common measure of visual acuity is the Snellen eye chart. Normal acuity is having 6/6 vision.


One who requires eyeglasses or contact lenses for in-focus vision.


Any imperfection in refractive state of the eye. Examples would be one with hyperopia, myopia, or astigmatism.


A non-rotationally symmetrical lens element that distorts image size & shape in one axis more than the other, because of its barrel (cylindrical) shape. Panavision lenses used for wide-screen movies contain anamorphic elements to “squeeze” the image, later “de-squeezed” by the projection lens.


A difference in refractive power of the two eyes in which the variance is at least one diopter.


A condition in which the surface of the cornea is not spherical, but bulges more in one axis than the other, like the back of a spoon. An astigmatic cornea causes light images to focus on two separate points in the eye, creating a distorted image and poor focus. Contact lenses designed to correct for astigmatism are called Toric-lenses.

Aqueous humor

Clear fluid that flows between and nourishes the crystalline lens and the cornea. It is secreted by the ciliary processes.

Barrel distortion

A distortion in which straight lines not passing through the center of the image bend outward (away from the center of the image). The curvature becomes more pronounced further from the center. This is the opposite of pincushion distortion.

BCVA (best corrected visual acuity)

The best possible vision a person can achieve with corrective lenses measured in terms of Snellen lines on an eye chart.

Chromatic aberration (same as color fringing)

The failure of a lens to bring light of different colors to the same focus.

Ciliary muscles

The muscles that alter the shape of the crystalline lens, thereby focusing the eye.


One of two types of specialized light sensitive cells (photoreceptors) in the retina that provides the ability to see objects in color and at high resolution in the central field-of-view. See rods.

Convergence / divergence

The turning of the eyes inward/outward so that they are both “aimed” toward the object being viewed.


The clear, dome-shaped “window” at the front of the eye that covers the iris and pupil. The cornea plays an important role in vision because it provides approximately 70 percent of the eye’s light-focusing power. Contact lenses rest on the corneal surface.

Crystalline lens

The natural lens of the eye, located behind the iris, which helps focus rays of light on the retina. The original state of the lens is transparent, but the lens may become cloudy with age (cataract). The lens has the ability to vary its shape, thereby focusing on objects closer than optical infinity.


A measurement of the refractive power of a lens element, as a portion of a metre.


One with 6/6 vision, without using corrective lenses.


A condition in which light rays focus correctly on the retina, without using corrective lenses; same as 6/6 vision.

Excimer laser

Used in PRK (photorefractive kerototomy) and LASIK (laser-assisted intrastromal keratoplasty) to reshape corneal curvature by ablating, or burning off, eye tissue.


Common term for hyperopia.


An eye disease characterized by narrowing of one’s field-of-view, caused by increased pressure within the eyeball. If not diagnosed and treated, glaucoma may lead to optic nerve damage, loss of visual field, gradual vision impairment, and sometimes blindness.


LONGSIGHTED, inability to focus on close objects. This occurs when one’s eyeball is too small, short or flat for the focusing system of the eye, or when the eye’s focusing mechanism is too weak (not enough positive diopter), thus causing light rays to focus behind the retina, making close objects appear blurry. A positive diopter lens is required to achieve normal vision.


The tissue behind the cornea that gives color to the eye (blue, brown, hazel, etc.). It controls the amount of light that reaches the retina, and the depth of focus of the eye, by varying the size of the pupil.

LASIK (Laser Assisted In-Situ Keratomileusis)

A vision correction surgery procedure to treat hyperopia, myopia and astigmatism. The curvature of the cornea is altered by first raising a “flap” of cornea, ablating the tissue underneath with an excimer laser (PRK) machine, then closing the flap on top. Sometimes referred to as “flap-n-zap”.


For presbyopic individuals (need reading glasses) who wear contact lenses. For example, with one who is nearsighted, a stronger power contact lens (for distance vision) is worn on the dominant eye, while a weaker power contact lens (for close vision) is worn on the non-dominant eye. The brain ignores the fuzzy image from one eye, and concentrates on the high-resolution image from the other eye.


SHORTsighted; enhanced ability to see close objects, inability to focus on distant objects. This occurs when one’s eyeball is too long or too big for the focusing system of the eye, or when the eye’s focusing mechanism is too strong (excess positive diopter), thus causing light rays to focus in front of the retina, making far objects appear blurry. A negative diopter lens is required to achieve normal vision.


With age, we all lose our ability to focus on nearby objects (accommodation). In our eyes, the crystalline lens loses flexibility and our ciliary muscles weaken. This limits our minimum focusing distance. Presbyopes with 6/6 vision need reading glasses; presbyopic myopes and hyperopes need bifocals. Contact lens users can choose between monovision, modified monovision or progressive contact lenses to achieve both near and far focusing ability.

PRK (photorefractive keratotomy)

A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma.

Progressive lenses

Lenses of multiple zones of optical power that provide both near and far focusing ability for presbyopic vision. For spectacle lenses, this is an alternative to bifocals or trifocals (elimination of tell-tale “lines” that separate lenses of differing powers). For contact lenses, these are sometimes referred to as “bifocal” contacts. But they are actually concentric rings (think “bullseye target) of varying powers that provide simultaneous near and far focusing ability. The brain ignores the fuzzy part of the image and concentrates on the high-resolution image.


The black, circular opening in the center of the iris, which is the colored portion of the eye.


The bending of light rays by the use of a lens or other refractive material (i.e. water).

Refractive index

A measure of a clear substance’s ability to slow photons, and thereby bend the direction of travel of off-axis rays of light. The denser the material, the more it will slow photons, and therefore bend the direction of travel.

Refractive error

In human vision, a defect in the ability of the eye to focus an image accurately. Common errors are astigmatism, hyperopia and myopia; half the world’s population requires some kind of vision correction.


Like the film in a camera, the retina (made of rods and cones) at the back of the eye receives images formed by the eye’s optical system, and sends impulses to the brain through the optic nerve.

RK (radial keratotomy)

Oldest vision correction surgery technique to treat nearsightedness. Flattens the curvature of the cornea by placing micro-incisions that resemble the spokes of a wheel in the tissue around the central optical zone.


One of two types of specialized light sensitive cells (photoreceptors) in the retina that provides both the ability to see objects in dim light (night-vision), and peripheral vision. However rods provide monochromatic images; in very low light, humans see objects as shades of gray.


The middle tissue layer of the cornea, comprising about 90 percent of corneal thickness.

Visual acuity

Clearness of vision. The ability to distinguish details and shapes of objects; also called central vision.

Visual axis

The central area of the cornea, pupil, and lens that light passes through to reach the retina and be “seen”.

Vitreous humor

The transparent, colorless mass of gel that lies behind the lens and in front of the retina.

For details of our associate practice in Edinburgh please visit – Trotters Opticians.