Astigmatism is the optical term for more than one point of focus. It occurs when the surface of the cornea or crystalline lens is not spherical. Light from an object may focus on the retina in one direction but not the other.
An anastigmatic eye has curves that are steeper in one direction than the other, like a rugby ball. A normal, spherical eye has curves that are the same in all directions, like a football. An anastigmatic person will have distorted vision.
Positioning a cylindrical lens in front of an astigmatic eye will refract the light in one direction more than in the other. Note how the light passes through the cylindrical lens in two different directions. The power of the lens is not the same all over. It is stronger in certain areas to compensate for the irregular curves on the astigmatic eye. When corrected, an astigmatic person will then see distance and close objects clearly.
Hyperopia means long sight and is where the image is formed behind the retina. This could be because the eye is too short, or the cornea or crystalline lens does not refract the light enough.
A hypermetropic person has blurred vision when looking at objects close to them, and clearer vision when looking at objects in the distance.
By placing a convex lens in front of a hypermetropic eye, the image is moved forward and focuses correctly on the retina. A hypermetropic person will then see distance and close objects clearly.
Myopia means short sight and is where light from an object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens refracts (bends) the light too steeply.
A myopic person has clear vision when looking at objects close to them, and blurred vision when looking at objects in the distance.
By placing a concave lens in front of a myopic eye, the image moves back to the retina so it will be clear. A myopic person can then see distance and close objects clearly.
Most people notice spots or floaters in front of their eyes. They are especially noticeable when looking at a plain white background and appear as greyish specks in our vision.
The vitreous fluid, the jelly inside the eye, contains small particles. These are either present from birth or are formed when the vitreous starts to deteriorate. These cast a shadow onto the retina at the back of the eye, which appear as spots and floaters.
Presbyopia comes from the Greek for “old sight”. It most commonly occurs when people reach the age of 40 and over.
As one gets older the crystalline lens becomes less flexible and therefore cannot change shape as easily. This means it can”t focus close images onto the back of the eye.
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