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REFRACTIVE ERRORS

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REFRACTIVE ERRORS: The term refractive error, describes the type of error in the focusing system of the eye, for example, near-sighted or far-sighted.  Images are seen clearly if they are sharply focused onto the retina.  The process of accommodation keeps images focused on the retina and the image remains clear whether it is being viewed from close-up or from a distance.  Errors in the focusing mechanism are referred to as refractive errors and they can be corrected by spectacles. 

There are three major forms of refractive error, in addition to presbyopia: 

MYOPIA: In this condition, also referred to as near-sightedness, the image is focused in front of the retina.  This can occur either because the eyeball is too long or because the lens system (the cornea and the crystalline lens) is too strong, bending the light rays so much so that the image comes to a focus in front of the retina. - This results in a blurred image. 

 
Myopic Eye
 

When something close-up is being observed however, the image is focused further back onto the retina and can then be seen clearly. 
Onset of myopia often begins in early childhood and continues to increase into the mid-twenties. 

Because people with myopia can see near objects well without their glasses, they may not need reading glasses as they age, or if they do need reading glasses it is usually at a much later stage than their contemporaries who never wore spectacles, but now require reading glasses to read the newspaper. 

Myopia can be inherited and therefore particular attention should be paid to siblings when one of the children in the family has been diagnosed as being near-sighted. 

 HYPERMETROPIA OR HYPEROPIA: This is the most common refractive error and almost all infants are far-sighted.  As they develop into childhood, the far-sightedness usually diminishes. 

Where the myope had the image in focus in front of the retina, the hyperope has the image in focus behind the retina. This can occur because of a shorter length eyeball or a lens or cornea which doesn't bend the light rays sufficiently. 

 
Hyperopic Eye
 

Being far-sighted means that the lens has to accommodate, even for images at distance.  The accommodation ensures a sharp image but can tire the eye if objects nearby are viewed for a lengthy period of time, causing eye strain.  Hyperopes will also need reading glasses at an earlier age than people without refractive errors. 

 ASTIGMATISM: This condition can be characterized by blurred images at both distance and near.  The basic problem is that the curvature of the cornea is not the same in all directions. 

One can think of the astigmatic cornea as being shaped like a football or the back of a spoon, with the curvature being steeper in one direction and less steep in the other, whereas the normal cornea looks more spherical (like a basketball). 

 
Astigmatic Eye
 

Astigmatism can be associated with either myopia or hyperopia.  It can be corrected with cylindrical lenses in the spectacle correction. 

PRESBYOPIA: This is the term for the condition where one's arms progressively become too short to hold the newspaper at a comfortable reading distance.  The ability of the lens to converge light rays decreases with age and therefore the object being observed needs to be held at a greater distance from the eye than previously necessary. 

This usually begins from age 40 to 42 in the person without refractive error and increases with time.  In myopes it occurs at a later stage and may in fact never progress to the point of needing reading glasses. 
The myope may need to remove his or her regular glasses to be able to read though.  The hyperopic person needs reading glasses earlier than persons without refractive error.
 
 If you have any further questions regarding refractive errors,
please consult your eye specialist.
 

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