REFRACTIVE
ERRORS
An eye
that , which does not have any refractive errors, and has the ability
to see the far and near and distant objects clearly, is called an
emmetropic (normal) eye. A refractive error means that the shape of
your eye does not bend (refract) the light rays
correctly.
In other words, the eye can not focus images properly, resulting in
a blurred image. In an emmetropic eye parallel rays of light from
a distant object and divergent rays of light from a near object focus
on the retina after being refracted by the cornea and the lens. The
eyes, having refractive errors have problems with focusing light rays
on the retina. Myopia (nearsightedness), hyperopia (farsightedness),
astigmatism and presbyopia are names given to different refractive
errors.
The refractive errors are quantified with the power of the lens that
corrects the refractive error. The unit of the lens power is measured
in diopters. 1 Diopter (D) is equal to the optical power of the lens
which has a focusing distance of 1 meter. A lens with 1 diopter power
focuses parallel light rays at 1 meter while a lens with 2 diopters
power focuses parallel light rays at 0.5 (1/2) meter.
Total focusing power of the eye is between 58 and 71 diopters. Due
to this amazing focusing ability the eye can see images from 25 cm
to infinity by focusing on a tiny screen (retina), which is only 17
mm away from its lens.
