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
(Fig. 1).
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 (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.
(Fig. 1).
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| Fig. 1. Refraction in an emmetropic eye. |
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 (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.
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