Our Eyes and Vision

83% of information transmitted to the brain is through sight. But how? Let’s look at the basic anatomy of the eye and how our eyes work.

Our eyes receive information through light waves, essentially colours. The human eye is a complex structure that focuses light reflecting from objects and turns it into electrical signals to send to the brain. The brain then converts the signals into the sensation we know as “vision”.

The eye is made up of the following major parts, each of which is explained briefly below.

At the front of the eye is a transparent structure called the cornea. It is responsible for the majority of the focusing power of the eye.

Behind the cornea is the iris, a coloured tissue that defines someone’s eye colour, but it’s responsible for more than just the way a person appears. The iris expands or contracts, controlling the amount of light entering the eye.

Did you know that the iris is unique to each individual? More so than the corresponding fingerprints on each hand, as the iris is different between the two eyes too.

The iris forms a circular opening at its centre, known as the pupil.

Between the cornea and the iris is an area called the anterior chamber, that is filled with clear fluid (the aqueous humour) which helps to provide nutrients inside the eye.

The white part of the eye is known as the sclera, which provides a tough protective coat to the layers inside the eye.

Attached to the sclera are the extraocular muscles which control the movement of the eyeball and its ability to change direction of gaze.

Behind the iris is located the crystalline lens. This lens is the only variable optical element in the eye, as it has the ability to change its shape via small muscles around the lens. This allows a person to change focus and see clearly both in the distance and up close. This process is referred to as accommodation. As we age, the lens material loses its elasticity, becomes harder, and the ability to focus on close objects will decrease.

After the light is focused by the cornea and the lens, it is transmitted to the retina. The retina covers almost the entire inner surface of the eye. It is made up of three layers of nerve cells, including light receptors. These cells are responsible for turning light into electrical signals which are then sent to the brain via the optic nerve.

In the centre of the retina is an area known as the macula, which is the most sensitive part of the human retina and is responsible for all fine detail vision.

Emmetropia

Emmetropia is a word used to describe a normal visual condition. A person is called an Emmetrope when no form of visual correction is needed, as the light entering the eye comes to perfect focus on the retina, the image is clear and precise.

This is the “normal” situation for people who do not have to wear glasses.

Just as in other parts of the body, there is variability in the size of the eye and its components. This variation makes for a range of optical defects.

Refractive Error

A refractive error is the inability of light rays to be focused sharply on the surface of retina.  Light, or objects, not focused sharply result in blurry vision.

Types of refractive errors include near-sightedness (myopia), long-sightedness (hyperopia), astigmatism, which can cause blurred vision both at distance and near, and presbyopia, gradual age-related long-sightedness.

Refractive errors are recorded in terms of a unit known as a dioptre, which describes the amount of power needed to focus an image precisely. The determination of a patient’s refractive error is one of the critical roles an Optometrist performs to generate a prescription for glasses or contact lenses.

Near-sightedness (Myopia)

Myopia is a refractive error that causes blurred distance vision and occurs because the image of an object is formed in front of the retina.

This is usually due to the fact that the length of the eyeball has increased. It is very rare at birth and generally develops in pre-adolescent years or thereafter. The length of the eyeball stops growing around the age of 25 and after that a person’s myopia tends to stabilize.

Myopic people often report difficulty driving (especially at night), reading the board at school, seeing the television clearly, but generally have clear near vision. Headache and eyestrain are rare but can occur due to squinting to see objects in the distance.

Most myopic eyes are healthy that are just simply longer and larger than other eyes. Normal myopia does not cause blindness.

Corrective optical lenses work by repositioning the image of the object onto the retina.

Long-sightedness (Hyperopia)

Hyperopia is a refractive error is a common visual problem causing blurred near vision because the lights rays are focused behind the retina.

Hyperopia occurs because the length of the eyeball is smaller than average.

Hyperopic people complain about difficulty reading a book and suffer from eyestrain and headaches especially after doing close work. Not everyone with hyperopia report blurred vision but may experience symptoms of visual discomfort, tiredness, burning and stinging eyes, or poor concentration.

Treatment for hyperopia depends on the person’s age, activities and occupation. Some only need glasses for close work, others may need to wear them full-time, depending on the amount of hyperopia, accommodation ability and how much of the day spent with close work.

Corrective optical lenses work by refocusing the image of the object onto the retina.

Astigmatism

Astigmatism is a very common refractive error and almost everybody has some astigmatism. Small amount seldom requires correction, but larger amount will cause blurred vision both in distance and in near and requires visual aid.

Astigmatism is usually caused by an irregularly shaped cornea, although occasionally it’s the crystalline lens that is the culprit. Astigmatism is caused when the cornea has an oblong shape, like a football, rather than a spherical shape, like a basketball. This oblong or oval shape causes the light to focus at two points on the retina, instead of a single point focus.

Astigmatism can be hereditary but may also be caused by eye accidents and extreme amounts of astigmatism may be seen in certain eye diseases.

Astigmatism can also occur together with myopia or hyperopia.

Corrective optical lenses that have more power in one direction than the other. By having different powers in each axis of the lens, light becomes focused in one focal point on the retina.

Presbyopia

Presbyopia literally means “aging eyes” and refers to the age-related changes we experience when trying to focus on close work. To focus on near objects, the crystalline lens must change its shape by the muscle tissues to bend light rays more and focus them clearly on the retina. When we are young this lens is a very flexible material and changes shape easily. As we age, the lens loses its flexibility and it becomes more difficult to focus on close work.

Around the age of 40-45, objects at a person’s reading distance become blurry, threading needles or fine handiwork becomes difficult, reading books is easier when held further away.

This process of the lens hardening continues until a person is in their sixties, when there is very little flexibility left in the crystalline lens. As this progresses, people begin to notice that objects in the middle distance are becoming blurry as well, making it difficult to read the computer screen or see prices in the supermarket.

There are different corrective lenses for presbyopia. Reading glasses will sharpen close vision but will make distance viewing blurry. When someone requires both distance and near vision correction, bifocal, trifocal or progressive (multifocal) lenses provide solution.