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Spectacles
work by re-focussing rays of light entering the eye so that the retina,
which is the part of the eye that receives the image, has clear and
un-blurred vision.
If the
image you see is out of focus, then you are either short or long sighted
and will need to correct, ie re-focus, your vision with spectacles
(or contact lenses).
An eye
examination (eye test) will reveal to the Optician what corrective
prescription is required.
The
prescription is then created in the lenses of the spectacles so that
your eye, retina and therefore your brain can receive a clear image.
There
are four types of corrective lenses that may be prescribed.
Single-vision
lenses
If you
need spectacles and are under 40, it is likely that your prescription
will be for lenses to correct your vision for one distance only. This
might be for short-sightedness (myopia)
or far-sightedness (hyperopia),
or to correct astigmatism.
Bifocals
and trifocals
After
the age of 40, you might begin to notice a problem when viewing objects
close to your eyes, a condition called presbyopia. To enable you to
continue to see at both near and far distances, you might be prescribed
bifocal or trifocal lenses.
Bifocals
contain a visible segment in each lens to correct for near vision.
This can be set above or below the distance segment depending on lifestyle
needs.
Trifocals
have a third segment to enable you to see objects at intermediate
distance as well as near and far.
Varifocal
or progressive lenses
In varifocals,
the visible lines between the segments on bifocal and trifocal lenses
are removed and replaced with a lens that provides a continuous range
of vision from near to far distance.
Reading
glasses
If you
don't need spectacles to correct for myopia
but are experiencing presbyopia (seeing objects at near distance),
a simple pair of reading glasses might be appropriate. These are usually
designed to allow you to look down through the lenses at objects close
to you, then to glance over the top of the spectacles to see distant
objects.
Want
to know more about lenses? ·
Origins
of the lens ·
A brief history of spectacles
What to expect from an eyesight
test or eye examination in the UK and Northern Ireland
What your spectacles
prescription means
Myths about spectacles
About sunglasses
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