Short-Sightedness (Myopia)

 

With myopia objects in the distance are more blurred than those at near.   This is because the light entering the eye is being bent too much and focuses in front of the retina.  Minus (concave) powered glasses and contact lenses or surgical techniques can be used to diverge the light and refocus it correctly on the retina.

 

Long-sightedness (Hypermetropia)

 

This often is misunderstood (especially when children are required to wear glasses to correct it).  In hypermetropia the light entering the eye is not bent enough and focuses behind the retina.  In this case, objects that you are looking at can be difficult to see in terms of the eye effort required, but not necessarily blurred. 

This is because the degree of difficulty and/or blurred vision you may have depends on the distance you are trying to focus on, the amount of hypermetropia you have and lastly your age-related focusing reserve.  When we are younger our eyes have a large reserve of positive focusing ability.  As an object comes nearer to us the lens inside the eye can change shape (become stronger and bend the light entering the eye more) to ensure the image focuses on the retina.  This focusing reserve can be also be used to “overcome” hypermetropia.

If you have a level of hypermetropia that is either greater than your focusing reserve or the amount of focusing power required for the distance you are trying to focus at, then you will experience blurred vision.  If your hypermetropia is less than the focusing effort required then you may not experience blurred vision, but you will likely suffer the effects of the focusing effort required such as eyestrain and headaches.

Astigmatism

 

Astigmatism occurs when light is bent differently as it passes through the eye.  The cornea (the front window to the eye) is usually the source for astigmatism.  The cornea of a normal eye has the same degree of roundness in all areas (that is the curve is equal in all 360 degrees).   An eye with astigmatism has a cornea that is curved unevenly with some areas that are steeper than others.  For example there may be a certain curvature at 90 degrees  (this is vertical in eyes) and another steeper curve along 180 degrees (horizontal in eye terms).  These two curves bend the light entering the eye unevenly and result in two focal points going towards the retina.  Astigmatism will blur images at all distances as there is no distance at which these two focal points will ever both be correctly focused on the retina.

 

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