About eye sight problems

We have heard frequently of some common eye-sight problems like myopia, hypermetropia and astigmatism. What exactly are these problems? I always get confused with long-sightedness and short-sightedness.

Hyperopia, also known as hypermetropia or colloquially as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or when the lens cannot become round enough), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves towards the eye, the eye must increase its power to keep the image on the retina. If the power of the cornea and lens is insufficient, as in hypermetropia, the image will appear blurred.

Hyperopia is often confused with presbyopia, another condition that frequently causes blurry near vision. Presbyopes who report good far vision typically experience blurry near vision because of a reduced accommodative amplitude brought about by natural aging changes with the crystalline lens. It is also sometimes referred to as farsightedness, since in otherwise normally-sighted persons it makes it more difficult to focus on near objects than on far objects.

Myopia, or nearsightedness also known as short sightedness, is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.

Those with myopia typically can see nearby objects clearly but distant objects appear blurred. The opposite defect of myopia is hyperopia or "far-sightedness" or "long-sightedness" — this is where the cornea is too flat or the eye is too short.

In optics, astigmatism is when an optical system has different foci for rays that propagate in two perpendicular planes. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. That is from a particular distance the person can only see either the horizontal or the vertical lines. She can’t see both at the same time from the same place.