Astigmatism is an optical defect which blurs the vision. Before learning what causes this defect, let us understand how the eye perceives light. When light enters a perfectly spherical lens (spherical like a basketball), it refracts or bends, evenly. If the lens is not completely spherical, the light that enters the lens is refracted more in one direction than another.
What is Astigmatism?
Now, under normal circumstances, when light enters a person’s eye which is naturally shaped like a sphere it creates a clear view of the object as the light is refracted evenly. However, if the eye of a person is curved but not completely spherical, like the back of a spoon, what happens? For this person, when light enters the eye it is refracted more in one direction than the other, allowing only part of the object to be in focus at one time. This is called a refractive error. Objects at any distance can appear blurry and wavy.
This defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus is called Astigmatism.
Types of Astigmatism
The eye defect is usually caused by an irregularly shaped cornea or distorted lens. When the cornea has an irregular shape, it is called corneal astigmatism. When the shape of the lens is distorted, you have lenticular astigmatism. As a result of either type, your vision for both near and far objects appears blurry or distorted. It’s almost like looking into a fun house mirror in which you appear too tall, too wide or too thin.
The steepest and flattest meridians of an eye with astigmatism are called the principal meridians.
It can further be classified as regular and irregular astigmatism. In regular, principal meridians are perpendicular to each other (90 degree separation). Opposite to this, in irregular, the principal meridians are not perpendicular to each other.
Most astigmatism is regular corneal, which gives the front surface of the eye an irregularly curvy shape, like that of a spoon. Eye injuries that result in scars on the cornea can cause irregular defect. Certain types of eye surgeries or a disease called keratoconus (gradual thinning of the cornea) can also cause irregular defect.
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Meanwhile watch an interesting video on human eye.