When the refractive power of the cornea and lens matches the axial length of the globe distant light rays are focussed on the retina, a situation known as emmetropia. If a non-accommodating eye is unable to bring parallel light rays from far objects into focus on the retina then it is called ametropia.
There are three basic conditions that produce ametropia:
This arises when there is a mismatch between the refractive power of the eye and the axial length of the eyeball. Excessive convergent power eg cornea too steeply curved (“refractive” myopia) or increased lens refractive index secondary to cataract (“index” myopia) causes distant light rays to be focussed too quickly in front of the retina. The same effect occurs if the refractive power is normal but the eyeball is excessively long (“axial” myopia).
A myopic individual is unable to see distant objects clearly without the aid of a concave lens eg glasses or contact lenses which diverge the light rays to allow the image to be focussed on the retina.
This arises when a mismatch results in distant parallel rays of light being inadequately focussed, the image is formed behind the retina. Either the corneal curvature is too flat (“refractive” hypermetropia) or the eyeball is too short (“axial” hypermetropia).
Younger hypermetropic individuals can increase the refractive power of the eye by accommodating (lens fattens) in order to see a distant object clearly. Older hypermetropic people, with no or limited accommodation, may be unable to see distant objects clearly and are likely to have very blurred near vision unless wearing convex lenses which help converge the light.
The cornea may not have the same radius of curvature (and therefore refractive power) in all directions, a condition known as corneal astigmatism. A rugby ball is an example of an astigmatic surface with two principle curvatures. Uncorrected optically this results in a blurred image on the retina as at least one curvature will be out of focus at any given time. Many myopic and hypermetropic patients will also have a degree of astigmatism.
This is a progressive loss of accommodative ability of the lens (due to loss of lens capsule elasticity) caused by aging. It usually becomes clinically significant around the age of 45yrs onwards as individuals notice difficulty focussing for near tasks. Presbyopia is independent of the distant refractive error and therefore occurs in people with myopia, hypermetropia and astigmatism. It is treated optically with convex reading lenses.