Normal Vision (Emmetropia)
In the human eye there are two
powerful focusing elements, the cornea (clear window to the eye) and the
crystalline lens. They have the ability to focus light on to the retina
(at the back of the eye) from all distances. When an object has to be
viewed close, such as reading a book, the lens changes shape and the
power of the eye increases bringing the print into focus. The lens
relaxes and decreases power to see objects further away. In some ways
the eye is similar to a camera. The cornea is the lens and the retina is
the film or image capture.
Short sight (Myopia), Nearsightedness
Occurs in over 25% of the
population. The length of the eyeball is too short and so objects are
blurred as light rays fall short of the retina. Corrective lenses will
place the image back on the retina, giving clear sight.
Long sight (Hyperopia), Farsightedness
Objects appear more blurred the
closer they are to the eye, as the length of the eyeball is too long
and hence light rays fall 'behind' the retina. Corrective lenses will
place the image back on the retina.
Is caused by a distortion in
the shape of the cornea. Normally the cornea is spherical like a soccer
ball, but an astigmatic cornea is like a rugby ball. Vision will appear
blurred at all distances, usually with accompanying short or long
sightedness. Similar corrective vision can be acheived as in other
defects of sight.
Affects all individuals from
the age of around 45 years. In presbyopia the lens loses some of its
elasticity and cannot change shape enough to focus on near objects. If
the person already has sight correction when presbyopia occurs then
varifocal glasses, contact lenses or separate pairs of glasses will be