Refractive errors are the main reason why we seek the services of an optometrist or ophthalmologist. They describe irregularities in the way the light rays are bent (refracted) in the eye. In a healthy eye the light rays are bent to focus exactly at one point on the retina, creating a sharp, clear image. This state of perfect vision is called “emmetropia”. Refractive errors are optical imperfections that prevent the eye from properly focusing light on the retina, causing blurred vision.
The most common types of refractive errors are myopia (nearsightedness), hyperopia (farsightedness) and astigmatism.
If you suffer from myopia, objects up close can be seen clearly; whereas distant objects appear blurry. Myopia is the most common eye condition in the world. In some areas almost half of the population is affected. Myopia, known commonly as nearsightedness, is a result of the eye having too much refractive power in relation to its length. In other words, the focal point of incoming light lies at a point in front of the retina, because:
This results in faraway objects appearing blurry, because the light is not focused on the retina, where the optic nerve transmits the image to the brain.
Unlike hyperopia, the prescription and refractive power of myopia is given in negative dioptre; the more negative the number, the more severe your refractive error. Depending on your level of myopia, you may only be able to clearly see objects that are a few centimetres away from you, and in more mild cases, a couple of meters away.
If you suffer from hyperopia, objects in the distance are usually seen as sharp; whereas close up objects appear blurry.
This condition, known as farsightedness, is a result of the eye’s shape being disproportional to its refractive power because:
The cornea and the natural lens normally refract incoming light rays to intersect at a single focal point that lies exactly on the retina, the layer of light-sensitive cells lining the back of the eye. A hyperopic eye refracts light to a focal point that lies behind the retina.
This causes vision problems, especially when trying to focus on objects at a close proximity. Blurriness occurs because the light is not focused on the retina, where the optic nerve transmits the image to the brain.
The prescription and refractive power of hyperopia is given in positive dioptre: the higher the number, the more severe your refractive error. Depending on your level of hyperopia, the distance you are able to clearly see varies between mild and severe cases.
In addition to myopia and hyperopia, many people also have an astigmatism. This condition occurs when the curvature of the cornea, the transparent part of the front of the eye, is uneven and slightly oblong – similar to an American football. In contrast, a normal cornea is evenly shaped all around like a basketball. Due to the unevenness of an astigmatic cornea, incoming light rays are refracted by the cornea in different angles.
Instead of one focal point like in normal vision, there are multiple focal points in an eye with astigmatism. The disarray of focal points fails to make a single, clear image on the retina, the layer of light-sensitive cells lining the back of the eye.
Without a focused image on the retina, the information sent to the brain through the optic nerve is distorted. This results in skewed or crooked images both near and far. Depending on the type of astigmatism, images appear stretched horizontally or vertically.