Hyperopia (farsightedness):

If you can see objects at a distance clearly but have trouble
focusing well on objects close up, you may be farsighted.
Hyperopia causes the eyes to exert extra effort to see close up.
After viewing near objects for an extended period, you may
experience blurred vision, headaches and eyestrain. Children
who are farsighted may find reading difficult.

Hyperopia is not a disease. It simply means that you have a
variation in the shape of your eyeball. The degree of variation
will determine whether or not you will need corrective lenses.
Hyperopia most commonly occurs because the eyeball is too
short; that is, shorter from front to back than is normal.
In some cases, hyperopia may be caused by the cornea having
too little curvature. Exactly why eyeball shape varies is not
known, but the tendency for farsightedness is inherited. Other
factors may be involved too, but to a lesser degree than heredity.

Hyperopia

 Myopia (nearsightedness):

If you can see objects nearby with no problem, but reading road
signs or making out the writing on the board at school is more
difficult, you may be nearsighted.

Myopia is not a disease.  It simply refers to a variation in the
shape of your eyeball.  Myopia most often occurs because the
eyeball is too long, rather than the normal, more rounded shape.
Another less frequent cause of myopia is that the cornea, the
eye’s clear outer window, is too curved.

 myopia

 Presbyopia:

Hold the book up close and the words appear blurred. Push the
book farther away, and the words snap back into sharp focus.
That’s how most of us first recognize a condition called
presbyopia, a name derived from Greek words meaning
“old eye.” Eye fatigue or headaches when doing close
work, such as sewing, knitting or painting, are also
common symptoms. Because it is associated with aging,
presbyopia is often met with a groan and the realization
that reading glasses or bifocals are inevitable.

As we age, body tissues normally lose their elasticity.
As skin ages, it becomes less elastic and we develop
wrinkles. Similarly, as the lenses in our eyes lose some
of their elasticity, they lose some of their ability to change
focus for different distances. The loss is gradual. Long before
we become aware that seeing close up is becoming more
difficult, the lenses in our eyes have begun losing their
ability to flatten and thicken. Only when the loss of
elasticity impairs our vision to a noticeable degree
do we recognize the change.

  presbyopia

 Astigmatism:

If you experience a distortion or blurring of images
at all distances, nearby as well as far, you may have
astigmatism. Even if your vision is fairly sharp, headache,
fatigue, squinting and eye discomfort or irritation may indicate
a slight degree of astigmatism. A thorough eye examination,
including tests of near vision, distant vision and vision clarity,
can determine if astigmatism is present. Astigmatism is not a
disease. It simply means that you have a variation or
disturbance in the shape of your cornea.

Astigmatism is one of a group of eye conditions known
as refractive errors. Refractive errors cause a disturbance
in the way that light rays are focused within the eye.
Astigmatism often occurs with nearsightedness and
farsightedness, conditions also resulting from refractive errors.

Astigmatism usually occurs when the front surface of
the eye, the cornea, has an irregular curvature. In
astigmatism, the front surface of the cornea is curved
more in one direction than in the other. With the cornea’s
shape more like that of an American football or rugby ball
than a basketball, the light hitting the more curved surface
comes to a focus before that which enters the eye through the
less curved surface. Thus, the light is focused clearly along
one plane, but is blurred along the other so only part of
anything being looked at can be in focus at any time.

This abnormality may result in vision that is much like
looking into a distorted, wavy mirror. The distortion
results because of an inability of the eye to focus
light rays to a point.

Not all corneas are perfectly curved, just as sets of
teeth are seldom perfectly aligned. The degree of variation
determines whether or not you will need corrective eyewear.
If the corneal surface has a high degree of variation in its
curvature, light refraction may be impaired to the degree that
corrective lenses are needed to help focus light rays better.

The exact reason for differences in corneal shape remains
unknown, but the tendency to develop astigmatism is
inherited. For that reason, some people are more prone to develop astigmatism than others.

 astigmatism