In the normal, healthy eye, the cornea is clear, smooth and round and the lens is clear, allowing light rays to pass through unobstructed and be focused sharply on the retina. However, if the cornea is not as smooth, clear or round as it should be, or the eyeball is too long or too short, light rays will bend (or refract) at odd angles, leading to blurry or distorted vision. This inability to achieve sharp focus is called refractive error, and is the most common eye problem.
Types of Refractive Errors, their causes and symptoms:
Hyperopia or long-sightedness:
Normally images will be focussed clearly on the retina at the back of the eye. However if the lens in the eye is too thin or the eye is too short the focal point for the eye is too long. This is where the term ‘long-sightedness’ comes from. You may also hear the term hypermetropia used which is the same thing. It means that things in the distance will not be very sharp and things up close will be even more out of focus.
Young children have incredible focusing powers and can change the shape of their lens to overcome some of their long-sightedness. However due to the strain this puts on the muscles in the eye it is not sustainable for long periods and can cause strain and cause the eyes to tire easily. So, by correcting the long-sightedness we allow the muscles in the eyes to relax so that the eyes can see a clear image without having to strain.
Short-sightedness or myopia:
Normally images will be focussed clearly on the retina at the back of the eye. However, if the lens in the eye is too thick or the eye is too long the focal point is too short for the eye. This is known as short-sightedness or myopia. It means that things close to you are in better focus and objects appear blurred the farther away they are.
Correcting short-sightedness gives a clear image on the retina so things farther away come into focus.
In a normal eye with no refractive error all light rays meet at the retina to give a clear focussed image. An eye like this will have a round or spherical front surface. In an eye with astigmatism the front of the eye is more oval or rugby ball shaped. This means that light rays coming from certain directions will focus just in front of or just behind the retina causing a blurred image.
Astigmatism along the horizontal and vertical axes are the least visually disabling. However, astigmatism can occur in any direction affecting light coming in at different angles which causes even more blur.
Presbyopia is when your eyes gradually lose the ability to see things clearly up close. It is a normal part of aging. In fact, the word "presbyopia" means "old eye" in Greek. You may start to notice presbyopia shortly after age 40. You will probably find that you hold reading materials farther away in order to see them clearly.
How can be diagnosed?
To diagnose Refractive Errors, eye doctor will ask for you some tests like: