This is the part of the eye test where your Optometrist will determine whether you will be able to see better if they prescribe you spectacles. It is split into two main parts, known as objective and subjective refraction.
Objective refraction can be done in two main ways. One way is to use an "Autorefactor", which is a machine that will use light to get a rough estimate of your prescription. The other is a technique used by an Optometrist called Retinoscopy. This is done by the Optometrist shining some light into through your pupil, and watching the reflex that is produced by the retina at the back of your eye.
Once they have a rough idea of what prescription that you may need, they will move onto subjective refraction. During subjective refraction they will show you letters on a test chart, and maybe some other targets to look at. They will then put different lenses in front of your eyes and ask you if these make things better or worse.
If they decide that you need spectacles, you most likely have one of the 'refractive errors' listed below.
Myopia, or short sight occurs when light is focused in front of the retina and causes distance vision to become blurred. Near vision, however, is usually clear.
This commonly develops in childhood or adolescence and is often first noticed by children complaining of not seeing the board clearly at school.
Hypermetropia, or long sight occurs when the natural tendency is to focus light beyond the retina rather than on it, and the eye has to make a compensating effort to re-focus. With a younger person this may only be possible with effort and may, for example, cause headaches when reading. In an older person, as well as making reading very difficult, it may also cause distance vision to become blurred.
Astigmatism occurs when the front of the eye is rugby ball shaped instead of football shaped. In this case both distance and near vision may be distorted. People with this condition may suffer from headaches or be unduly sensitive to light.
As we get older, the lens of the eye thickens and slowly loses its flexibility, leading to a gradual decline in our ability to focus on objects that are close up. This loss of focusing ability is called PRESBYOPIA.