Refractive Errors of the Eye

Light rays enter an eye without refractive error and pass through the cornea (the transparent anterior part of the external coat of the eye which covers the iris and the pupil, continuous with the sclera, and influences sharpening vision the most) and the lens where they refract and fall on the retina, onto the point producing the sharpest vision. Such an eye sees clearly with a well-defined sharpness. If the rays of light entering the eye refract in such a way as to fall either before or behind the retina, the eye is affected with a refractive error - nearsightedness, farsightedness or astigmatism. The most frequent reason for these errors is either a change in eye length or the parts of the eye which are most responsible for refraction of light rays (the cornea and the lens) refract the rays more or less than required.

Pic Normal eye:

Nearsightedness (myopia)

- is a refractive error caused when the eye is too long or refraction by its optic system is too great. That is why parallel rays entering the eye are focused in front of the retina, resulting into objects being seen only when they are close to the eye.

Farsightedness (hypermetropia or hyperopia)

- is a refractive error caused when the eye is too short or refraction by its optic system is insufficient. That is why parallel rays entering the eye are focused behind the retina resulting in distant objects being seen more distinctly than near ones, and the image formed on the retina is blurred and foggy.

Astigmatism

- is a refractive error of the eye in which parallel rays of light from an external source do not converge on a single focal point on the retina. It is caused by an unequally curved cornea, i.e. the cornea has no regular hemispherical shape but is flattened in some places and more curved in others. The image formed on the retina is distorted and blurred.

Presbyopia

- inability to focus sharply for near vision due to loss of elasticity of the lens of the eye; patients who are above 40 years of age and use glasses only for reading.