a
Accommodation
The ability of the eye to change its focus between distant and near objects.Acuity
The sharpness of central vision. Normal visual acuity is defined in relation to the Snellen acuity chart as 6/6 vision. The Snellen acuity chart is the eye chart used to establish the prescription for your glasses, featuring rows of letters that begin large at the top and get smaller as they proceed towards the bottomAnisometropia
A condition of the eyes where they have significantly unequal refractive power.Astigmatism
Structural defects of the eye in which the light rays from a viewed object do not meet at a single focal point, resulting in blurred images being sent to the brain. An astigmatic cornea is not perfectly rounded like a basketball, but has an irregular shape similar to the side of a football. Astigmatism is most often combined with myopia or hyperopia.
b
Binocular vision
Simultaneous use of the two eyes. Normal binocular vision yields a stereoscopic image and parallax-induced depth perception.
c
Cornea
Transparent tissue that forms the front part of the eyeball, covering the iris and pupil. The cornea is the first part of the eye that bends (or refracts) the light and provides most of the focusing power.
Cylinder
Refers to the degree of astigmatism (uneven roundness) present in the cornea.
d
Diopter
Unit of measurement for the refractive power of an optical lens (equal to the power of a lens with a focal distance of one meter). A negative diopter value (such as -3D) signifies an eye with myopia, and a positive diopter value (such as +3D) signifies an eye with hyperopia.
f
Farsighted
A common term for hyperopia.
FDA
Abbreviation for the Food and Drug Administration. This is the United States’ governmental agency responsible for the evaluation and approval of medical devices.
h
Hyperopia
Also called farsightedness, hyperopia is the inability to see near objects as clearly as distant objects, and the need for accommodation to see distant objects clearly.
k
Keratoconus
An inherited corneal disease. The cornea gradually becomes thinner and less able to maintain its shape against the pressure of the fluids inside the eye. It bulges forward, blurring vision, and may eventually require a corneal transplant.
m
Myopia
Also called nearsightedness or shortsightedness, the inability to see distant objects as clearly as near objects.
n
Nearsighted
Common term for myopia.
o
Orthokeratology
A non-surgical procedure involving using contact lenses to alter the shape of the cornea and effect a change in refractive errors.
p
Presbyopia
The inability to maintain a clear image (focus) as objects are moved closer. Presbyopia is caused by reduced elasticity of the lens with increasing age.
r
Refraction
Test to determine the refractive power of the eye; also, the bending of light as it passes from one medium into another.
t
Topography
A tool used to see refractive problems that might be present in the cornea. Corneal topography is used not only for screening all patients before refractive surgery such as LASIK but also for fitting contacts.
