The eye works like a simple camera. Light rays pass through the eye to focus on the retina at the back of the eye. This image is then transmitted to the brain.
The iris (coloured part of the eye) works like the aperture of a camera and regulates the amount of light entering the eye.
Myopia – Short Sighted
In a short sighted eye the light rays focus in front of the retina. This causes a blurred image at the retina. This occurs because the eye is too long or the cornea is too steep. Generally short sighted people can see images up close but have most diffi culty with objects in the distance.
Hyperopia – Long Sighted
In a long sighted eye the rays would come to focus behind the retina. This occurs if the cornea is too fl at or the eye is too short. Generally long sighted people can see objects better at a distance and have trouble with near objects and reading. However, sometimes they are blurred for both near and distance.
AstigmatismAstigmatism is when the curvature of the cornea is more like an egg rather than a ball. This causes the light rays to be bent in different directions as it passes through the cornea, the light can therefore cause a blurred image at the retina.
Astigmatism can occur on its own or more commonly in conjunction with short and long sightedness. Generally people with astigmatism have a slight blur at all distances.
Presbyopia
With increased age the natural lens inside the eye becomes less able to change focus. This translates to an inability of the eye to focus on near objects. This usually happens in the mid to late forties when reading glasses are needed.