Emmetropia & Ametropia
- Emmetropia (optically normal eye) can be defined as a state of refraction, where in the parallel rays of light coming from infinity are focused at the sensitive layer of retina with the accommodation being at rest.
- At birth, the eyeball is relatively short, having +2 to +3 hypermetropia. This is gradually reduced until by the age of 5-7 years the eye is emmetropic and remains so till the age of about 50 years. After this, there is tendency to develop hypermetropia again, which gradually increases until at the extreme of life the eye has the same +2 to +3 with which it started. This senile hypermetropia is due to changes in the crystalline lens.
Ametropia (Refractive error OR Refraction error)
- Ametropia (a condition of refractive error), is defined as a state of refraction, when the parallel rays
of light coming from infinity (with accommodation at rest), are focused either in front or behind the sensitive layer of retina, in one or both the meridians.
The ametropia includes myopia, hypermetropia and astigmatism.
- Hypermetropia (hyperopia) or long-sightedness is the refractive state of the eye wherein parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest .
Thus, the posterior focal point is behind the retina, which therefore receives a blurred image.
- Myopia or short-sightedness is a type of refractive error in which parallel rays of light coming from infinity are focused in front of the retina when accommodation is at rest.
- Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines.