Your eye has two structures with curved surfaces that bend (refract) light onto the retina, which makes the images:
- The cornea, the clear front surface of your eye along with the tear film
- The lens, a clear structure inside your eye that changes shape to help focus on near objects
In a perfectly shaped eye, each of these elements has a round curvature, like the surface of a smooth ball. A cornea and lens with such curvature bend (refract) all incoming light to make a sharply focused image directly on the retina, at the back of your eye.
A refractive error
If either your cornea or lens is egg shaped with two mismatched curves, light rays aren't bent properly, causing a refractive error. This makes a blurry image. Astigmatism is a type of refractive error.
Astigmatism occurs when your cornea or lens is curved more steeply in one direction than in another. You have corneal astigmatism if your cornea has mismatched curves. You have lenticular astigmatism if your lens has mismatched curves.
Either type of astigmatism can cause blurred vision. Blurred vision may occur more in one direction, either horizontally, vertically or diagonally.
Astigmatism may be present from birth, or it may develop after an eye injury, disease or surgery. Astigmatism isn't caused or made worse by reading in poor light, sitting too close to the television or squinting.
Other refractive errors
Astigmatism may occur in combination with other refractive errors, which include:
- Nearsightedness (myopia). This occurs when your cornea is curved too much or your eye is longer than normal. Instead of being focused precisely on your retina, light is focused in front of your retina, making distant objects seem blurry.
- Farsightedness (hyperopia). This occurs when your cornea is curved too little or your eye is shorter than normal. The effect is the opposite of nearsightedness. When your eye is in a relaxed state, light never comes to a focus on the back of your eye, making nearby objects seem blurry.
Source :- https://www.mayoclinic.org/diseases-conditions/astigmatism/symptoms-causes/syc-20353835
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