Refractive error is a term that refers to an optical imperfection that prevents light rays from coming to a sharp focus on the retina. Defects of refraction include hyperopia (farsightedness) myopia (nearsightedness) presbyopia (an age-related inability to focus on an object at close range) and astigmatism.
When light focuses in front of the retina because the eye is too long or the lens of the eye is too strong the result is a refractive error called Myopia (nearsightedness). Light that focuses in front of the retina results in blurry vision when looking at distance objects while near objects may be clear.
With an eye that is too short or has a lens which is not strong enough to bend the light rays sufficiently the light rays focus behind the retina. This condition commonly referred to as farsightedness can result in blurry distance and near vision.
Astigmatism is a common vision condition that produces blurred images. In a normal eye the cornea is round like a baseball. The light rays focus evenly on the retina at the back of the eye. In the astigmatic eye the cornea is egg shape preventing light from focusing correctly on the retina. The retina receives multiple images because of the irregularly shaped cornea (or lens of the eye) which produces blurred images.
Presbyopia occurs with age as the lens of the eye naturally becomes less flexible over time reducing the ability to focus on near objects. Presbyopia is a normal part of life (usually starting in the early 40’s) and happens to each of us to some degree. Once presbyopia sets in an eye needs a different correction for distance vision and for near. Presbyopes must wear glasses with bifocal trifocal or “no-line” progressive lenses.
What is 20/20 vision?
A Snellen chart consists of letters and numbers that your eyecare practitioner will ask you to read from a distance of 20 feet during an eye examination. A normal eye is defined as one that can just read a certain size type at 20 feet. A person that has 20/20 vision can read at 20 feet the same line of type that the person with normal vision can read at 20 feet. If you can read at 20 feet what a normal person could read at 60 feet then your vision is 20/60. The higher the number at the bottom of the fraction the larger the letters are on the Snellen chart. For children who are too young to read letters another chart using shapes helps determine their visual acuity. 20/20 vision indicates the clarity of vision at a distance and does not mean that vision is flawless as poor depth perception peripheral vision or colour vision can affect visual ability.
How to make sense of your Prescription
If you are nearsighted farsighted or have astigmatism you probably require either spectacles or contact lenses. A prescription is simply the power of the spectacle lens that will correct your refractive error.
Eye Sphere Cylinder Axis Reading Add O.D. -3.25 -1.25 170 +2.25 O.S. +0.50 -1.50 156 +2.25
The power of a lens is measured in units called Diopters. The amount that light rays will bend as they pass through the lens determines the diopter value. A high diopter number results in a strong prescription as there is more bending of the light rays. Conversely a lower diopter figure suggests a weaker prescription. A convex lens causes light rays to converge and its power (in diopters) has a plus sign. Convex lenses are thick in the centre and thin at the edges similar to a magnifying glass. The design works by bringing light rays towards a central point. A concave lens causes light to diverge and its power has a minus sign. Unlike the convex lens the concave lens is thin in the centre with a gradual thickening towards the edges. People who are farsighted need a convex lens (+ power lens) whereas people who are nearsighted need a concave lens (- power lens). On a prescription the wording oculus dexter (O.D.) refers to the right eye while oculus sinister (O.S.) refers to the left eye. Some eye health professionals will use the abbreviation ‘RE’ for the right eye and ‘LE’ for the left eye.
The ‘sphere’ is denoted by a plus ‘+’ or minus ‘-‘ symbol written before the diopter number. It refers to the degree of farsightedness or nearsightedness. For those with astigmatism a ‘cylinder’ power and ‘axis’ (orientation of the astigmatism measured in degrees) accompanies the prescription. When multi-focal lenses are required (for presbyopic patients) the word ‘add’ will accompany the prescription to indicate that there are two types of refractive errors in one eye.