How your eye works
Your eye works like a camera. The front of your eye, the cornea, iris, pupil, and lens focus the image onto your retina, which lines the inside of your eye. The retina is sensitive to light and acts like the film in a camera, capturing images and then sending them via the optic nerve to your brain where the images are interpreted.
Seeing the images clearly
There are two factors at work in seeing clearly. The first is proper focus. Just like a camera your eye needs to be focussed properly to send clear images to your brain. The second is healthy eye tissue. Any damage or disease to the cells within your eye has the potential to remove your ability to see fully – sometimes damage can be permanent.
Early detection prevents serious eye damage
If you have any problems with your eyes or with seeing clearly then you need to see an optometrist.
Any change in vision should be checked, It may be normal, or it could be the result of a more serious condition. Changes in vision could be related to vascular, neurological, or other medical conditions such as high blood pressure or diabetes.
Eye exams and vision screening
Vision screening (or sight testing) will only tell you if you would benefit from glasses. Vision Screening will not tell if your eyes are healthy. Not being able to see well is the most common sign of eye problems including eye disease that can cause blindness. This is why it is always necessary for an optometrist to examine your eyes thoroughly to make a diagnosis before prescribing treatment.
A comprehensive eye exam will measure any refractive error that you have and will also investigate for signs of eye diseases that have the potential to make you blind.
You can tell you are having a comprehensive eye exam if it includes assessment of:
- Medical history
- Slit-lamp examination
- External eye
- Internal eye
- Visual fields
- Subjective refraction
- Eye muscles
- Colour perception
- Peripheral fundus
- Macular health
It takes a while so you should expect to be in the chair for at least 30 minutes and maybe more. For more people that news will be that they have great eye health. Only around 60% of people examined will need to be prescribed glasses or contact lenses for refractive error.
Refractive error is the term used to describe poor focus. It may be caused by the shape of the eye and is resolved by applying a prescription lens in front of the eye to ensure a clear image reaches the retina. There are a number of specific refractive problems which cause poor vision including myopia, hyperopia, astigmatism.
Myopia causes blurred vision. The most common problem for those who are shortsighted is difficulty seeing distant objects clearly. Some myopic children have never had clear distance vision and so are not aware of what they are missing. School vision screening will often detect myopia. Frowning and screwing up the eyes in an effort to see better is common and this may cause headaches. Shortsighted children will sometimes hold reading material quite close. In their mid to late forties most people with perfect distance vision need reading spectacles, however many people with myopia can see close work clearly without them.
A young "normal" eye looking into the far distance sees clearly without making any focussing effort. A hyperopic young eye looking into the far distance can only see clearly if the muscular focussing system inside the eye is used.
A normal eye begins to use its focussing system as an object comes closer. A longsighted eye has to make the same focussing effort for near work as a normal eye, but this is in addition to the effort it makes to keep distance vision clear. With hyperopia, a long sighted eye has to "work harder" than a normal eye at all distances.
Astigmatism is a very common focusing error of the eyes which causes blur. It is caused by the shape of the eye, usually due to the cornea's surface or occasionally due to the eye's lens being tilted. Sometimes astigmatism can be inherited but it often happens as a normal characteristic growth.
Blur from astigmatism is not like that from myopia where all of an image is equally blurred, because some parts of the image are more out of focus than others. The blur of astigmatism makes things uncomfortable to look at, difficult to focus and may cause headaches, tiredness and poor concentration.
Normal, healthy, young eyes have a wide range of focus from far distance to a few centimeters. In a young eye the lense is very flexible. Muscles within the eye have the ability to change the shape of the lens and by doing so, change its focus. This change happens so quickly that we don't even realise our eye is refocusing!
As we get older, the lens of the eye thickens and slowly loses its flexibility, making it difficult to hold objects very close and clearly. Around the age of 40 to 45, vision at our normal reading distance becomes blurry. We have to hold print further away to avoid tired eyes.
The loss of focusing ability is called presbyopia. It is not a disease, but a normal change which affects everyone. Presbyopia doesn't occur suddenly. It doesn't affect distance vision. It is a change which begins in adolescence and cannot be prevented.
Floaters (or spots) are particles which float inside the eye and cast shadows on the light sensitive tissue at the back of the eye (the retina), reducing vision. Usually nothing more than a nuisance, floaters can results from eye disease or injury and need to be assessed by your optometrist.
Glaucoma usually comes without any warning. Glaucoma is an eye disorder in which the fluid pressure inside the eye causes progressive damage to parts of the optic nerve. The pressure usually increases when there is inadequate drainage of fluid from inside the eye. A gradual but permanent loss of vision occurs unless the condition is treated.