The ability of the eye to change its focus from distant to near objects (and vice versa). This process is achieved by the crystalline lens changing its shape.


Amblyopia also known as lazy eye, is an early childhood condition where a child's eyesight in one eye does not develop properly. Amblyopia occurs when one eye experiences a blurred view and the other one a normal view, but the brain only processes the normal view.
The term lazy eye is inaccurate, because the eye is not lazy. In fact, it would probably be more accurate to say lazy brain, because it is a developmental problem in the brain, not an organic problem in the eye.


Age-related macular degeneration. AMD is a disease that damages the macula, the central part of the retina, leading to a loss of central vision and leaving only the peripheral or lateral vision intact. AMD sufferers begin to have trouble distinguishing colors and see straight lines as if they were deformed.


Characterizes all eyesight disorders that prevent a clear image from forming on the retina, with the exception of age-related presbyopia. Myopia, hyperopia and astigmatism are all forms of ametropia.


Anisometropia means that the two eyes have a different refractive power, so there is unequal focus between the two eyes. It is the second most common cause of amblyopia.

Anti-Reflective Coating

Anti-reflective coating (also called AR coating or anti-glare coating) is a microscopically thin multilayer coating that eliminates reflections from the front and back surface of eyeglass lenses. With reflections eliminated, lenses with AR coating provide better vision for night driving and more comfortable vision for reading and computer use. It makes lenses nearly invisible so people can focus on the eyes, not distracting reflections from eyeglasses.


Astigmatism is a very common vision condition that causes blurred vision due either to the irregular shape of the cornea or the curvature of the lens inside the eye.
Most people have some degree of astigmatism. Slight amounts of astigmatism usually don't affect vision and don't require treatment. However, larger amounts cause distorted or blurred vision, eye discomfort and headaches.

Bifocal lens

A bifocal lens is created with two different areas of vision correction, which are divided by a distinct line that sits horizontally across the lens. The top portion of the lens is used for distance, while the bottom portion of the lens is used for closer vision.


Blindness is strictly defined as the state of being totally sightless in both eyes.


A cataract is a clouding of the crystalline lens of the eye that impair vision.
Cataracts are the most common cause of vision loss in people over age 40 and are the principal cause of blindness in the world.


Lens coatings can enhance the performance and appearance of your eyeglass lenses. Several types of coating can be applied to corrective lenses: scratch-resistant, anti-reflective, polarizing, coloring, antistatic, anti-smudge.

Cone cells

Cone-shaped photoreceptive neurons located in the retina used for central vision and color perception.


Transparent front part of the ocular globe shaped like a spherical, or slightly domed, cap. In combination with the crystalline lens the cornea plays an important role in focusing images on the retina.

Corrective lenses

Corrective lenses are designed to correct eyesight disorders. The corrective lens is a combination of material, optical surface and coatings.

Crystalline lens

Transparent biconvex optical lens located behind the pupil. The crystalline lens refracts light to focus images on the retina. The aging of the crystalline lens is at the origin of presbyopia.


A diopter is the power of the lens that is needed to correct your vision to normal, the higher the number the stronger the lens.


Double vision, also known as Diplopia, is the simultaneous perception of two images of a single object. It is a symptom to take seriously.

Distance Vision

Vision of objects situated either at infinity or more usually at some 5 or 6 meters.


Emmetropia describes an eye that lacks visual defects. This means the image that is formed on person’s retina is clear and precise. The eye with emmetropia doesn’t require contacts or eyeglasses, it is sometimes described as perfect vision. It is the opposite of ametropia.


The Eye-Sun Protection Factor (E-SPF) is an international index certifying the overall UV protection provided by a lens. It is an objective rating system developed by Essilor International which can rate any lens for daily eye wear as well as for sunglasses.
Inspired by the cosmetic industry standard of UV protection, the E-SPF rating system follows a simple pattern: The higher the E-SPF, the better the UV protection.


Increase in intra-ocular pressure resulting, if left untreated, in an irreversible deterioration of the optical nerve and of the retina, as well as an alteration of the visual field, i.e. a reduction in visual performance, often accompanied by headaches and aching eyes.

Half-eye glasses

Half-eyes are the smaller "Ben Franklin" style glasses that sit lower down on the nose. They allow you to look down and through the lenses for near work, and up and over them to see in the distance


Hyperopia, or far-sightedness, is an eyesight defect due to an eye that is too short and/or insufficiently powerful. The image forms behind the retina, which explains why a person with hyperopia thus sees badly in near vision but well in far vision.
Hyperopia is the opposite of myopia.

Intermediate Vision

Vision of objects situated beyond 40cm from the eye but closer than, say, 1,5 meter

Intraocular pressure

Is the fluid pressure inside the eye, it is an important aspect in the evaluation of patients at risk from glaucoma.


Circular membrane that delimits the pupil. The iris acts as a diaphragm that contract according to the intensity of light. The pigmentation of the iris determines the color of the eyes.


Keratoconus is a progressive eye disease in which the normally round cornea thins and begins to bulge into a cone-like shape. Keratoconus can occur in one or both eyes and often begins during a person's teens or early 20s. As the cornea becomes more irregular in shape, it causes progressive nearsightedness and irregular astigmatism creating additional problems with distorted and blurred vision. Glare and light sensitivity also may occur.


Central part of the retina. The macula is composed uniquely of cone cells directly exposed to rays of light and enables the precise vision required for reading or for the recognition of details. This is the area with the maximum activity in the eye.

Mid-distance lenses

Mid-distance lenses are intended for presbyopic subjects; these are lenses designed for near vision but with an extended depth of field.


Myopia (near-sightedness) is an eyesight disorder caused by an eye that is too powerful, or too long. The image forms in front of the retina; a person with myopia thus sees badly in far vision but well in near vision.

Near Vision

Vision of objects situated 25-50 cm from either the eye.


A repetitive involuntary movement of the eye whose direction, amplitude and frequency is invariable.

Ocular globe

Spherical organ (the eyeball) that receives vision. It consists of three layers, the sclera, the uvea and the retina and their content: vitreous humor, crystalline lens and aqueous humor.


Physician specialized in the treatment of eye diseases and conditions, and in the correction of eyesight disorders. Ophthalmologists may also carry out corrective surgery.

Optical aberration

An optical aberration is a distortion in the image formed by an optical system compared to the original.

Optical addition

The optical power (of a lens) required for near vision, in addition to that required for far vision.

Optical correction

The most common method of correcting vision is optical correction using eyeglasses or contact lenses, which correct refractive errors by refocusing light rays on the retina to compensate for the shape of your eye. An eye exam will determine your need for vision correction or a change in your correction.

Optical power

Optical power defines the ability of a lens or contact lens to correct a visual defect. Optical power is measured in diopters.

Optical surface (or design)

Surface on which either a reflection or refraction of the light is produced. The optical surface gives the material its ability for optical correction. The number of optical surfaces is almost infinite: they may be single-vision, bifocal or progressive.


Optician is trained to dispense and fit spectacles and other optical aids, working from the prescriptions written by optometrists and ophthalmologists. They advise patients on various types of lenses and spectacle frames.


In many English-speaking countries, Optometrists are eyecare specialists who dispense refractive examinations. Optometrists do not deal with pathologies or surgery.


Branch of ophthalmology treating eyesight disorders by means of re-education and eye-training sessions. Orthoptics is a paramedical profession exercised by a medical aide, the orthoptician.

Peripherical Vision

Vision resulting from stimulation of the retina outside the fovea or macula.

Photochromic lenses

Photochromic lenses are eyeglass lenses that darken automatically when exposed to sunlight, then fade back when you return indoors. Other generic terms sometimes used for photochromic lenses include "light-adaptive lenses" and "variable tint lenses."

Polarized Lenses

A polarized lens is a quality sun lens that not only reduces bright light from the sun, as regular tinted sun lenses do, but also eliminates dazzling polarized light thanks to a very thin polarizing film inserted inside the lens, playing the role of a Venetian store. This results in a better clarity of vision, a truer color perception, and a greater visual comfort.

Polarized light

Polarized light is an intense bothersome reflected glare that causes discomfort for the eye, and which appears when the sun’s rays hit a smooth, horizontal surface. This is particularly common in areas where reflected light is intense (sea, mountains, road surfaces etc...).


Eyesight disorder caused by the aging of the crystalline lens, which with time thickens and loses its suppleness. As the crystalline lens becomes more rigid, it changes shape less easily and the subject sees less and less well in near vision. Everybody over the age of forty suffers from presbyopia.

Prescription laboratories

Production units that transform semi-finished lenses into finished lenses with the precise characteristics of the order. The custom work carried out by the laboratories enables us to provide the very large number of optical combinations required by wearers, especially as regards the correction of presbyopia. The laboratories are responsible for surfacing (grinding and polishing) and coating (coloring, anti-scratch, anti-reflective, anti-smudge etc.) the lenses.

Prismatic Effect

The change in direction imposed on a ray of light induced by an ophthalmic lens when the eyes look in various gaze directions (except through the optical centre). The prismatic effect at each point on the lens has an impact, in the central vision area, on the wearer's oculomotor strategy.

Progressive lenses

Progressive lenses, sometimes called "no-line bifocals," are designed to correct presbyopia by varying optical power progressively. They provide the ability to see at all distances, including at arm's length for computer use and up close for reading. With progressive lenses, you can look up to see clearly across the room and in the distance. You also can look ahead to view your computer in the intermediate zone and drop your gaze downward to read and do fine work comfortably through the near zone of the lenses.


Central opening of the iris through which rays of light enter the eye. The diameter of the pupil varies according to the ambient light level.

Pupillary Distance (PD)

The distance between the centres of the pupils of the eyes. PD measurement is used to ensure proper lens placement monoculary. It is a major measurement in dispensing progressive lenses.

Readers or ready-made glasses

Ready-made glasses are inexpensive eyewear that are sold without prescription. They contain lenses with an equal power for each eye, and they are primarily used for the correction of presbyopia (reading glasses). They are suitable for people who spend a great time concentrating on material close-up. But if you try to look up and across the room through the reading lenses, everything appears blurry.


Change in the direction of propagation of a ray, determined by variations in the speed of propagation. Term used to qualify the optical examination.

Refractive index

Used to characterize the capacity of a transparent optical material to refract light and produce an optimal correction. The higher the index is for the same correction, the thinner the lens.


Device used to automatically measure the refraction of the eye.


Light-sensitive membrane at the back of the eye on which object images are formed and which transmits information to the brain. This hypersensitive membrane plays an essential role in the perception of light, colors, details, shape and movement.


Thin, cylinder-shaped cells located in the retina that react to light but which are incapable of distinguishing colors. Rods are highly sensitive and ensure vision in very low light conditions. Retinal rods are external extensions of the rod cell, which are neurons located in the retina.

Single vision lenses

Single vision lenses are used to correct ametropia. They may also be used for the correction of presbyopia but far vision will be blurred. The power is the same over the entire surface of the lens.


Strabismus is a visual problem in which the eyes are not aligned properly and point in different directions. One eye may look straight ahead, while the other eye turns inward, outward, upward, or downward. It is a common condition among children.

Visual acuity

Visual acuity describes the acuteness or “sharpness” of vision; that is the ability to perceive small details.

Visual fatigue

Visual fatigue is characterized by a smarting or acute irritation of the eyes, blurred vision and headaches, most often at the end of the day.

Vitreous humor

Transparent and vitreous gel contained between the pupil and the retina, located behind the crystalline lens.