The Human Eye

The Normal Eye

The eye is similar to a camera in design. It is composed of an optical lens and an imaging apparatus.

Lens

There are 2 optical apparatus in the eye. The external cornea and the internal crystalline lens. The cornea is the main objective lens. It accounts for 75% of the refractive power of the eye. The young lens inside the eye is the focusing apparatus. Our brain can use muscles inside the eye to change this lens’s shape to allow change in focus from far to near (accommodation) for reading vision.

Imager

The retina is the imaging apparatus of the eye. The retina is analogous to the film or CCD chip in a camera. With a normal retina and corrected optics a normal person can see a size 6 print from 6 metres hence 6/6 vision (20/20 vision in inches). Some young people can see even better than this standard.

eye anatomy emphasising cornea, natural lens and retina

eye-camera compare eye to camera

Refractive Error

In the normal focused eye there is no need for spectacle correction. Light is sharply focused onto the retina at the back of the eye, such that there is clear distance vision.

However, almost half of the population do not have clearly focused vision without optical aids. Approximately 25% of people are myopic (short-sighted) and 20% hyperopic (long-sighted). This group of people require some form of vision correction such as spectacles, contact lenses or surgical vision correction.

Over 95% of people after age of 55 the internal crystalline lens no longer changes focus. This condition is called presbyopia. These patients need optical assistance from multifocal spectacle, contact lens or presbyopia surgery for reading.

normal vision


Myopia, Near Sight

Myopia is where the eye is too long and light is focused in front of the retina. Distant objects are blurred but near objects are seen clearly. Spectacles or contact lenses are required for distance vision correction. Treatment for myopia is the most common refractive surgery treatment.

myopic vision - short sightness


Hyperopia, Far Sight

Hyperopia is when light is focused behind the retina due to insufficient optical power. Symptoms from hyperopia is more complicated than myopia.

In a young hyperopic person, the eye uses increased internal crystalline lens focusing power through accommodation to compensate. A young hyperopic person often has completely clear near and distant vision.

With age, the magnitude of this compensation (accommodation) diminishes and reading glasses are needed at a relatively early age (see below).

Later in life distance glasses are also needed. By the age of 50 most hyperopic patient will need glasses for near and far vision. Laser and lens surgery can also correct hyperopia.

hyperopic vision - long sightness


Astigmatism

Astigmatism is where the eye does not focus light evenly, usually due to the a patients’ cornea being more oval shaped than it should be (rugby ball shaped rather than football shaped). Astigmatism can be corrected by glasses, contact lenses or refractive surgery.

astigmatism vision


Presbyopia

The ability of the eye to change focus (accommodate) deteriorates with age. By the age of 45 most patients would notice some impairment of near vision. By the age of 55 most patients would be dependent on reading glasses for reading. This is mainly due to progressive changes in the compliance in the internal natural lens. Refractive surgery such as multifocal lens implant and mono vision laser surgery can alleviate the symptoms of presbyopia.

presbyopic vision


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