Short & long sightedness
Short-sighted and long-sighted are common vision disorders caused by the abnormal growth of the eyeball, which in turn disrupts the focus point and length in the eye. The focus point is located on the retina and is the image that is sent to the brain. The focal length is the distance between the lens and focus point. A baby’s eyes are already about two-thirds the size of an adult’s eyes, which can grow disproportionally with age for many reasons including genetics, premature birth, lifestyle factors and other eye diseases. Having a longer or shorter than normal eyeball alters the focal length, leading to a blurry image at the focus point.
In order to see things in-focus, the light rays must travel through the lens of the eye and converge at the focus point, at the back of the eye, on the retina. The point of light ray convergence is called the focal plane. So in order to see things clearly, the focal plane must be on the retina (focus point). If the focal plane is not on the retina, images will be blurry. When we want to focus on objects close-up, the lens can adjust its focus power to ensure that the light rays converge in the shorter distance so the focal plane is still on the retina. Therefore the two main factors of focus are the length of the eye and the lens.
Myopia is the clinical name given for near/short-sightedness. Objects that are far-away appear blurry in short-sighted individuals because their eyeballs are too long or their lenses are too strong. This results in the focal plane occurring in front of the retina rather than on it. Short-sighted people see things clear close-up and the lens may not even need to adjust because the light rays are able to converge (given the longer focal length) at the retina. Therefore, a short-sighted individual can see well close-up, but not far-away. This can be corrected by wearing concave lenses (curved inward) that spread the light rays further apart to extend the focal length so the focal plane can reach the retina, as shown in the diagram.
Hyperopia is the clinical name given for far/long-sightedness. Objects close-up appear blurry in long-sighted individuals because their eyeballs are too short or their lenses are too weak. This results in the focal plane occurring behind the retina rather than on it. Objects far-away appear normal because the focal plane can be brought forward by the lens onto the retina. Closer objects require greater focus and the demand in long-sightedness is beyond what the human lens can provide, so the focal plane remains behind the retina. This can be corrected by wearing convex lenses (curved outward) which bring the light rays closer together to shorten the focal length so the focal plane is on the retina, as shown in this diagram.
Comparing The Two
Short-sighted people can see clearly close-up, but cannot focus on anything far-away. Long-sighted people can see clearly far-away, but cannot focus on anything close up.
Getting Your Eyes Checked
If you are experiencing any vision changes or eye strain, you should see a doctor. The degree of short- or long-sightedness can vary significantly between individuals, hence why people have different prescriptions of glasses and it is important to get the right prescription. Some people can have mild cases of short- or long-sightedness, in which they can get by OK without glasses or may not even realize they have a visual problem. However over time, the strain this places on the eyes together with natural age-deterioration of eye function may require that individual to get glasses in their later years despite not having any problems in their early years.
A Snellen Eye Chart will be used by your eye doctor for diagnosis. Glasses and contact lenses do not cure the problem but enable you to see much more clearly and thus improve related symptoms by alleviating blurry vision and headaches resulting from the use of strained, sore eyes. Treating severe far and short sightedness is important because these eye disorders can increase the risk of developing a lazy eye. Besides wearing corrective eyewear, laser eye surgery is also available, which involves permanently changing the shape of the cornea.