How Your Eyes Work

The eye works like a camera. Incoming light rays are bent by the cornea and the lens inside the eye which focuses them accurately onto the retina. This is similar to the way a camera focuses an image onto a film. The retina receives the image which is transmitted to the brain via the optic nerve.
A refractive error occurs when the image formed on the retina is blurred. The three main types of refractive error are myopia, hyperopia and astigmatism.
The use of glasses and contact lenses enable us to bend the incoming light to focus at the correct point on the retina.

Normal vision

Clear vision is achieved when the cornea and the lens focus all light onto the retina (the back of the eye).


Myopia (short sightedness)

Myopia or short-sightedness is where the light rays focus in front of the retina because the eye is too long or the focusing power of the cornea and/or lens are too strong.

Typically without glasses the near vision is clear and the distance vision is blurred.


Hyperopia (long sightedness)

Hyperopia or long-sightedness is where the light rays focus behind the retina because the eye is too short or the focusing power of the cornea and/or lens is too weak.

Typically without glasses the distance vision is clearer than the near vision, but both may be affected.



Astigmatism is a blur caused by an uneven shape of the cornea (the front window of the eye). One direction of the cornea is steeper than others, preventing light from focusing to a single point, therefore causing blurry vision.



Presbyopia is a diminishing of the focusing power that occurs due to age and results in difficulty reading. Typically it occurs after 45 years of age. It is similar to long-sightedness but the distance vision is unaffected.


What does 20/20 vision mean?

Vision is measured at a specific distance and is recorded as a fraction.
20/20 is the old term for average vision – as it is a measurement in feet (although this is still commonly used in the USA as they still measure in feet and inches). These days we measure in metres, with the equivalent fraction of 6/6.
The first number tells us the distance you are standing from the vision chart. The second number tells us the what you can see.
For example, if your vision is average you should see at 6 metres what the average person sees at 6 metres, and this is recorded as 6/6.
If your vision is better than average you may see from 6 metres away what the average person would see from 4 metres, and your vision is recorded as 6/4.
If your vision is worse than average, you may, for example, see from 6 metres what the average person sees from 18 metres. This would be recorded as 6/18.