There are three components that determine how the eye is focused:

All three components must be within normal values and correlate with each other in order for the eye to be in perfect focus. This condition of perfect focus is called emmetropia. If one or more components are outside of normal values, the eye will be out of focus for distance vision, near vision, or both.

Components that determine how the eye is focused
Myopia

Also called nearsightedness or shortsightedness. Distance vision is out of focus, but near vision is in focus. This typically occurs because the eyeball is longer than normal, but can also occur if the lens or cornea is more powerful than normal.

Myopia typically develops in childhood or adolescence, and increases progressively during the growing years. This is due to continued lengthening of the eyeball as the body grows.

Myopia is corrected with minus (-) lenses. See how by moving your mouse over the image below.

Myopia
Hyperopia

Also called farsightedness or longsightedness. This typically occurs because the eyeball is shorter than normal, but can also occur if the lens or cornea is less powerful than normal.

Hyperopia forces the eye to focus harder than normal to see clearly. In a young person with good focusing ability, vision may still be clear at distance and/or near due to this extra focusing. However, this fatigues the eye's focusing muscles, causing eyestrain or headaches, an inability to read for long periods of time, or difficulty changing focus from distance to near and vice versa. As we get older, the focusing ability of the eye progressively decreases. As this occurs, distance and/or near vision becomes out of focus.

In children, hyperopia is normally present from birth, and usually remains the same or decreases through the growing years. Over the age of 40, many adults develop hyperopia, which often continues to increase with age.

Hyperopia is corrected with plus (+) lenses. See how by moving your mouse over the image below.

Hyperopia
Astigmatism

Astigmatism is distortion in the shape of the cornea or lens, causing it to be more curved in one meridian and less curved in the meridian 90° away. For example, the cornea may be more curved vertically than it is horizontally. If the amount of astigmatism is high enough, both distance and near vision are out of focus. Astigmatism can also cause doubling or shadowing around objects.

Astigmatism can be present from birth, or develop in childhood or adolescence, and can increase, decrease or remain the same throughout life.

Astigmatism is corrected with cylindrical lenses (also called toric, particularly in contact lenses). See how by moving your mouse over the image below.

Astigmatism
Presbyopia

Starting in the teen years, our eyes gradually lose their ability to focus over time. This is due to the progressive growth of the eye's lens, making it increasingly dense and inflexible, and therefore unable to change focus.

For most people, this doesn't become a problem until the early or mid-forties, when near vision becomes blurred. The loss of focusing ability typically progresses over a 15 - 20 year period, until the late 50s or early 60s.

Presbyopia is corrected with plus (+) lenses. See how by moving your mouse over the image below.

Presbyopia