Focusing Errors
Focusing errors are due to light rays from objects
not focusing on the retina. This happens because there is a mismatch
between the length of the eyeball and it’s lens power.
Spectacle lenses work by shifting the light rays so that they come
to a focus on the retina.
Normal Sight / Emmetropia
 Distance
& near are both good
Light
rays focus on the retina
Short Sight / Myopia
Distance
vision is blurry – glasses are required for TV, driving, the
board at school
Near
vision is generally OK, although may be blurry in very high degrees
of myopia
The
light rays come to a focus in front of
rather than on
the retina.
Caused
by the eyeball being too long or the eyes lens being too strong.
Far Sighted / Hyperopia
Near
vision is blurry
Distance
vision generally OK, although may be blurry in high degrees of hyperopia
The
light rays come to a focus behind
the retina, rather than on it.
Caused
by the eyeball being too short or the eyes lens being too weak.
Astigmatism
Can
affect distance, near or both
Light
rays from different planes are focused at different points
Caused
by the cornea not being perfectly spherical
Occurs
in conjunction with short or long sight.
Astigmatism Explained
In
astigmatism different parts of objects are out of focus by different
amounts. For example if someone is just shortsighted with no astigmatism,
then when they look at a cross both the horizontal and vertical limbs
are blurred by the same amount. If somebody with astigmatism looks
at the cross, they will see the vertical limbs more in focus than
the horizontal ones or vice versa.
How
Is Astigmatism Corrected?
Different
powers are required in different meridians of the lens. In the example
above, if the vertical limbs of the cross are more out of focus, then
the glasses will require a stronger strength in their vertical meridian,
compared to the horizontal meridian.
Presbyopia
This
is a form of long sightedness that occurs from mid 40’s onwards
Near
vision is blurry
Distance
vision is sharp
Caused
by a loss of accommodation
What
is Accommodation?
The eye is designed
to allow one to focus at different distances. It does this by reshaping
its own internal lens and hence changing its power. The process is
called accommodation.
How
is the Lens Reshaped?
The lens
is attached to a muscle. Contraction of the muscle will make the lens
thicker and more curved, increasing it’s power. When the muscle
relaxes the lens becomes thinner and less powered.
To focus for close objects the lens needs to be made fatter to provide
more power.
Why Does Accommodation
Loss Occur?
In a young person the
lens is very soft and jelly like and easy to reshape. Throughout our
lives, the lens gradually hardens. This process speeds up in early
middle age until by the mid-late forties it has become so hard that
the muscle cannot easily re-shape it. At this point spectacles must
be used to provide the focusing power that the eye can no longer provide
itself.
Isn’t it
Possible to Strengthen the Eye Muscle?
No,
the muscle that re-shapes the lens is made from a different type of
muscle fibre than that of arm or leg muscles. This sort of muscle
will not be made stronger or bulk up if made to work against a greater
resistance such as a more rigid lens. It will simply tire and give
up.
Will Wearing Reading Glasses Make My Eyes Deteriorate Further?
No. The hardening of the lens is due to internal
biological cellular activity. This activity is not affected by external
factors such as what you do or what you wear on your nose.
Further Information:
More information is available by clicking these links:
Myopia: http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=259§ionId=14003
Hyperopia: http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=532
Astigmatism: http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=37§ionId=10202
Presbyopia: http://www.eye-care.org.uk/item_view.php?item_id=99&content_id=4
|
| |
| |
|
|
