Book Now

+ 961 1 759637

Visual Errors

The human eye suffers from visual error (scientifically termed “refractive error”) when it cannot transmit a sharp image out of the objects we see. Light rays stemming from the objects around us enter the eye through clear cornea, then through the pupil and the lens. These light rays are focused on to the retina, a light sensitive tissue lining the back of the eye. Signals from the retina are sent through the optic nerve to the brain, where they are interpreted as the images we see. 70% of the eye’s focusing power comes from the cornea. When people have problems seeing clearly, it is frequently because of a refractive error.


There are four main types of refractive errors:

1. Myopia (nearsightedness)

2. Hyperopia (farsightedness)

3. Astigmatism

4. Presbyopia.

People with refractive errors might have one or a combination of these conditions.

Visual errors explained (English)
Types of visual errors

Myopia is also known as nearsightedness. This often occurs when the distance between the cornea and retina is too long, light rays entering the eye are focused in front of the retina, causing the image that falls on the retina to be blurred. Myopia blurs vision for far, while near vision focus becomes even closer. Myopia typically starts in childhood and adolescent age, but can start anytime later. It tends to increase at variable rates, but usually stabilizes between 18 to 21 years of age. Myopia however can still progress at any age later. A general consensus, endorsed by the National Institute of Health, is that there is yet no effective way to stop the progression of myopia. Timely correction of myopia using eyeglasses or contact lenses has not been found to halt progression. Prevention of accommodative spasm in children, whether by wearing bifocal reading glasses or through taking frequent breaks during near focused activities like reading and computer work, might be effective. Also lack of outdoor play might play a role. The prevalence of Myopia is 30-40% in the United States and Europe, and up to 70-90% in some Asian countries.

Myopia visual simulator
Myopia versus hyperopia:
schematic diagram
Myopia visual simulator

Hyperopia or hypermetropia is also known as farsightedness, and often occurs when the distance between the cornea and the retina is too short. Light rays entering the eye have not yet come into focus when they reach the retina. So again the images are blurred. Hyperopia blurs both the near and the far, but more so the former. Many young patients can accommodate and compensate for the hyperopic error they have, sometimes at the expense of headache, especially upon reading or working on computers. Routine screening at school might miss it. With aging, accommodation weakens and the hidden hyperopic error (latent hyperopia) starts to gradually appear and increase (becoming "manifest hyperopia"), affecting first the near, then later the far. 

Mild hyperopia visual simulator
Moderate hyperopia visual simulator

Astigmatism is a condition where the cornea is curved unevenly. A cornea that is curved the same in all directions is shaped like a basketball or a soccer ball, while a cornea with astigmatism is more curved in one direction than the other, like a rugby ball or American football. Light passing through this uneven cornea is not properly focused on the retina. Astigmatism can be coexistent with myopia (myopic astigmatism), hyperopia (hyperopic astigmatism), or both (mixed astigmatism).

Astigmatism visual simulation

Presbyopia is a normal condition associated with age. It causes problems with our near vision. When we are young, the lenses in our eyes are flexible and are able to change focus easily between near and far objects. As we age, this flexibility gradually decreases. At around age 40, many people begin to need corrective lenses for reading. 

Presbyopia: schematic diagram
Presbyopia visual simulator