Errors of refraction

In a normal eye focused for distant objects, parallel rays of light come
to a sharp focus exactly on the retina. It can accommodate for clear vision of
objects from infinity (far point) down to 25 cm (near point). This ideal
refractive state is called emmetropia. A deviation from emmetropia is called
ametropia. The important forms of ametropia are myopia,
hypermetropia, astigmatism, presbiopia. Ametropia results from an
imbalance between the length of the eye ball and its refractive power. (Errors of refraction)

Myopia (Short Sightedness)

Myopia results if the lens curvature is too great or the entire eyeball
becomes elongated. Light rays entering the eye are refracted more than is
necessary. Consequently light is focused in front of the retina. The image
perceived is thus blurred. The condition is called short-sightedness as
objects near the eye are clearer than those further away. Myopia can be
corrected by placing a concave lens in front of the eye. The surface of the
concave lens refracts light rays in such a way that the rays diverge slightly
from their original path. The lens of the myopic eye now refracts the
diverged light rays in to focus on the retina. (Errors of refraction)

Hypermetropia (Long Sightedness)

Hypermetropia results when the curvature of the eye lens is not great
enough. Light rays are not refracted enough and would thus be focused
behind the retina. The condition is called long-sightedness because distant
objects are clearer than near ones. This happens because light rays from
distant objects require less refraction than rays from near objects.
Correction of hypermetropia requires placing a convex lens in front of the
eye. The lens converges light rays before they enter the eye so that the eye’s
focuses the light correctly on the retina. (Errors of refraction)

Astigmatism

Astigmatism occurs if either the cornea or lens is distorted. One part
of the focusing mechanism then refracts light rays too much, or too less.
Usually most of the images perceived is out of focus. Light rays from part of
the object are focused in front of the retina, as in myopia. Rays from other
parts would be focused behind the retina, as in hypermetropia. Astigmatism
can be corrected by placing a lens in front of the eye. The curvature of this
lens varies from one part to another to compensate for the eye’s
deficiencies. (Errors of refraction)

Presbiopia

This is the result of a reduction in the amplitude of accommodation
with age due to hardening and loss of plasticity of the lens. Hence it becomes
less capable of being moulded into a more complex form. Presbiopia begins
at about 40 years of age. The remedy is convex lenses for reading. Any
defect in the eye should be consulted immediately with the optometrist. (Errors of refraction)

Errors of refraction

Myopia (Short Sightedness)

Hypermetropia (Long Sightedness)

Astigmatism

Presbiopia

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