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Refractive errors
This category includes several common disorders:

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Colour deficiency ("colour blindness")

An inability to recognise certain colours or groups of colours, for example the inability to distinguish between red and green. This is not a disease, but an inherited condition which cannot be treated or corrected.

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Eye injuries
These happen most often during play or sports activities. Because the overwhelming majority of these incidents can be prevented, teaching staff have a special responsibility to:

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Presbyopia (Greek (πρέσβυς) "presbys", meaning "old person") describes the normal process of ageing, during which the natural lens of the eye loses the ability to focus on near objects.

Similar to grey hair and wrinkles, presbyopia is a symptom caused by the natural course of aging. The first symptoms are usually first noticed between the ages of 40-50.

Symptoms
The first symptoms most people notice are, difficulty reading fine print, particularly in low light conditions, eyestrain when reading for long periods, blur at near or momentarily blurred vision when transitioning between viewing distances. Many advanced presbyopes complain that their arms have become "too short" to hold reading material at a comfortable distance. When they perform near work, such as embroidery or handwriting, they may have headaches or eyestrain or feel fatigued.

Presbyopia, like other focus defects, becomes much less noticeable in bright sunlight. This is not the result of any mysterious 'healing effect' but just the consequence of the iris closing to a pinhole, so that depth of focus, regardless of actual ability to focus, is greatly enhanced, as in a pinhole camera which produces images without any lens at all. Another way of putting this is to say that the circle of confusion, or blurredness of image, is reduced, without improving focusing.

A delayed onset of seeking correction for presbyopia has been found among those with certain professions and those with miotic pupils. In particular, farmers and housewives seek correction later, whereas service workers and construction workers seek eyesight correction earlier.

Thus normally sighted people who have never needed glasses before may require reading spectacles like bifocals or multifocals starting in their forties.

Does this mean my eyesight is deteriorating?
Not necessarily. Presbyopia is not an illness, and once the effects have been neutralised with corrective glasses, you should see close objects as clearly as you did before the focusing problems started.

What causes presbyopia?
When you focus on a close object, a muscle in the eye contracts to change the shape of the lens. Over time, the lens loses its elasticity until it becomes virtually impossible to focus on objects close by.

Can presbyopia be prevented?
While some treatments claim to be able to delay the onset of the condition, it will eventually be inevitable.

How is presbyopia diagnosed?
A comprehensive eye examination by an optometrist includes a test for the quality of your near vision. This will reveal the extent of any presbyopia, if this is in fact the cause of your complaint.

Will it get worse?
Presbyopia develops from childhood, and while it may only be detected around the age of 40, it will probably continue to develop until about 65. After that age, major changes in vision are unlikely.

How can presbyopia be treated?
Prescription glasses such as reading glasses will restore your focusing ability. A comprehensive eye examination is essential, however, to eliminate other possible causes of near-sightedness.

Will I always need glasses for reading?
Probably yes. Because the condition continues to change until quite late in life, you will need a new prescription every two or three years. Your optometrist will recommend the best option.

Will presbyopia affect my distance vision?
No. If you are wearing prescription glasses for distance vision, your optometrist may prescribe new combination glasses (i.e. bifocals) to eliminate the need to repeatedly change spectacles for close work.

Are contact lenses available for presbyopia?
Contact lens technology continues to develop, and contact lenses are now available to correct most vision conditions, including presbyopia. Your optometrist will be able to provide details.

How regularly should I have my eyes examined?
Once a year. If you’re over 40, an annual check-up will not only provide you with the correct prescription lenses, but it also reveals the presence of any potentially dangerous eye diseases.

Will presbyopia affect my lifestyle?
Once you’ve adjusted to your reading glasses or other eyewear, you’ll find that life can continue virtually as before. Because presbyopia is not a disease, it should not affect your well-being.

Many patients with myopia also suffer from a degree of astigmatism, or ovalness to their corneas. Astigmatism occurs when the cornea is shaped more like a rugby ball than a soccer ball, leading to distorted or tilted images as a result of the unequal bending of light rays entering the eye. Patients suffering severe astigmatism have blurred vision for distant as well as near objects.

Hyperopia is the medical term for “far-sightedness” or “long-sightedness. It occurs when an eye ball is too short for the cornea’s curvature. Light entering the eye focuses behind the retina, producing blurred images.

As an object moves toward the eye, the eye must increase its power to keep the image in focus on the retina. In Hyperopia the power of the cornea and lens is insufficient and the image will appear blurred due to the eye’s inability to focus on near objects, and in extreme cases unable to focus on objects at any distance.

People with hyperopia can experience:

Hyperopia is detected with a vision test called a refraction. Young patients’ eyes require a cycloplegic refraction where the focussing muscle of the eyes are relaxed so they are unable to mask their farsightedness with accommodation.

Hyperopia is often confused with presbyopia, another condition that frequently causes blurry near vision. Presbyopes who report good far vision typically experience blurry near vision because of a reduced accommodative amplitude brought about by natural aging changes with the crystalline lens of the eye.

 

Myopia ( Greek: μυωπία - myopia meaning “near-sightedness") is the medical term for near-sightedness, which means the ability to see “near” objects clearly without glasses, while objects in the distance are blurry. It occurs when the eyeball is too long for the cornea’s curvature. Light rays entering the eye cannot focus sharply on the retina, but instead focus further forward, causing a blurred image.

There are varying degrees of myopia. The more myopic you are, the more blurred objects will appear. However, while a lot of people believe their myopia is severe because they are so dependent on glasses and contact lenses, only one in ten myopic people actually fall in the severe or extreme categories.

Degrees
Myopia, which is measured in diopters by the strength or optical power of a corrective lens that focuses distant images on the retina, has also been classified by degree or severity:

Nearsightedness may be corrected with glasses, contact lenses, Orthokeratology or refractive surgery. Depending on your vision problem, you may need to wear your glasses or contact lenses all the time or only when you need distance vision, like driving, seeing a chalkboard or watching a movie. If you're nearsighted, your prescription is a negative number. The higher the numeral, the stronger your lenses will be.

Presbyopia and the 'payoff' for the nearsighted
Many people with myopia are able to read comfortably without eyeglasses even in advanced age. Myopes considering refractive surgery are advised that this may be a disadvantage after the age of 40 when the eyes become presbyopic and lose their ability to accommodate or change focus.