What is the Lens of the Eye?

The lens of the eye is a relatively transparent structure, made of proteins and fluids, located behind the iris. It, along with the cornea, helps the eye focus light onto the retina. The shape of the lens is like a slightly deflated ball being stepped on, forming a roundish ellipsoid. The shape of the lens changes based on the surrounding muscles, allowing it to focus on things both close and far away.

In this photo the light (yellow lines) enters the eye and passes through the lens (marked with an arrow). The lens focuses the light on the retina at the back of the eye.

Parts of the Eye

The eye is round, but not an exact sphere. While you can see some parts of the eye when looking at it, the majority of the eye is hidden inside the skull.

The main parts of the eye are:

Cornea - covers the outside part of the eye, assists with focusing light on the retina
Pupil - center part of the eye, usually dark, dilates based on the amount of light
Iris - colored part of the eye, regulates how much light enters the eye
Lens - behind the iris, focuses the light on the retina
Retina - accepts the images in view, allows colors to be identified
Optic nerve - connects the eye to the brain allowing us to understand what we see

This diagram of the eye shows the location and shape of each part.

Eye Lens Function

The primary function of the eye lens is to bend the light to focus it on the retina. Light enters the cornea when the eye is open. The pupil and iris control how much light enters the eye and passes through to the lens. The lens works like a curved lens in a camera, allowing for focus, adjustment, and clarity. The bending of the light is called refraction.

There are muscles around the lens called the ciliary muscles that allow the shape of the lens to change slightly, so the eye can focus on things both near and far. The focused light is projected onto the retina. Of course, because the lens is curved, the projection is either upside down or backward. Finally, the image moves from the optic nerve to the brain where it is adjusted so that we think we are viewing it correctly.

Accommodation

Accommodation is the ability to quickly shift focus from something close to something far away. The ciliary muscles are responsible for the amount of accommodation. These muscles actually change the shape of the lens as they tighten and loosen around it. This is important when driving, as the driver needs to focus on something close (the car's speedometer) as well as things far away (the traffic light).

The majority of the lens is made of proteins. Interestingly, the lens doesn't have any blood flow. It has fluid, called the aqueous humor, between the cornea and lens. This fluid provides it with energy, cleans it, and drains the eye.

Sometimes the lens doesn't function properly. This typically falls into three different categories.

Common Vision Problems

Presbyopia

This affects the majority of people as they age, even if they have no other vision problems. It typically starts around 40 and stops progressing around 70 years old. Presbyopia is caused by the changes in proteins in the lens during aging. The proteins become thick and stop flexing as much and the ciliary muscles function less effectively. The result is difficulty focusing on nearby objects, but it can be easily corrected with glasses, contact lenses, or eye surgery.

Nearsightedness (Myopia)

Myopia, more commonly known as nearsightedness, happens when the shape of the eye or lens causes light to focus in front of the retina instead of on the retina. Someone who is nearsighted can see things near them clearly, but things farther away are often blurry. Some people are born nearsighted, but many develop it as a child or teen when the body grows and changes.

A person with myopia, or nearsightedness, can see things close to them clearly, but not things far away.

Farsightedness (Hyperopia)

Hyperopia, more commonly known as farsightedness, happens when the eye causes light to refract to a point behind the retina. Someone with farsightedness can see things that are farther away, but things close up appear blurry. Some of the first symptoms of developing farsightedness include headaches and eyestrain. Farsightedness affects around 10% of people.

Someone with hyperopia, or farsightedness, can see things far away clearly, but things close by are often blurry.

Eye Lens Conditions

There are many different medical conditions that can develop in the eye. These can impact the function of the lens and how well someone can see.

Age-related Macular Degeneration

This condition is related to aging and impacts the central part of the retina. It is the leading cause of permanent difficulty reading or viewing things close-up among people over 65 years old.

Amblyopia (Lazy Eye)

Better known as having a "lazy eye," amblyopia is a common vision impairment in children. It has many causes including strabismus or one eye being more nearsighted/farsighted than the other eye. The result is one eye is weaker than the other and the brain favors the stronger eye.

Cataract

Cataracts are the clouding of the eye lens. They can occur at any age, including birth, and are the leading cause of blindness globally.

Lesson Summary

The cornea and lens in the eye help focus light on a single point. The lens is made primarily of proteins and can change shape with the help of the muscles surrounding it. It enables people to focus on things up close and at a distance. The ability of the lens to change shape in order to alternate focus is known as accommodation.

Ideally, the focus point is on the retina, at the back of the eye. However, sometimes the focus point is before the retina, which is called myopia. Myopia is better known as nearsightedness because it means the person can see things that are near but has difficulty seeing things that are farther away. If the focus point is past the retina, it is called hyperopia, or farsightedness. This is when a person can see things far away, but things that are near the eyes are blurry.

Frequently Asked Questions

Where is the lens in the human eye?

In the human eye, the lens is just behind the iris. This is still considered the front part of the eye.

What does the lens of the eye do?

The lens bends the light so that it focuses on a single point, ideally on the retina. It can adjust and change shape to help focus on things both far away and near the eye itself.