How do glasses lenses correct vision?

• Myopia (near-sightedness), hyperopia (far-sightedness), and astigmatism are three different types of refractive errors – vision conditions where light is not focused accurately within the eye, causing blurred vision.
• The simplest way to correct refractive errors and restore clear vision is to wear prescription glasses or contact lenses.
• As different people require different prescriptions, prescription lenses can vary in size and shape, depending on the type and degree of refractive error being corrected.

The ability to see clearly and have good visual acuity depends on many factors. First, the eye needs to be healthy both inside and outside, from the clear tissues that provide the passage of light through the eye (e.g. the cornea and lens), to the tissues that help process light into vision (e.g. the macula).

The other requirement for clear vision is whether the eyeball’s components all match each other in shape in order to focus light in an optimal way – like the lenses of a camera. When light enters the eye through the front of the eye (the cornea), it travels towards the retina, the light-sensitive layer at the back of the eye. If light is precisely focused onto the retina, the image that we see is put into sharp focus.

A mismatch in the eye’s structures can cause light to be focused poorly, causing vision to be blurred and out of focus. This is known as refractive error, also known as the eye’s ‘prescription’. Different types of refractive error exist, near-sightedness (myopia), far-sightedness (hyperopia), and astigmatism – and the severity can vary. As eyes come in all shapes and sizes, refractive errors are extremely common.

Myopia, also known as near-sightedness, is a type of refractive error in which distant objects appear blurry while near objects can be seen clearly. This occurs when the eyeball’s overall focusing power is higher than required, and is usually associated with longer eye lengths or steeply curved corneas. As a result, light entering the eye is brought into focus too soon – in front of the retina instead of directly on it, resulting in blurred distance vision.

A simple way to understand how a person with myopia sees is to imaging taking a photograph in portrait mode. Although the subject of the photo (e.g. a person’s face) is focused clearly, the background of the image appears blurred.

To correct myopia, the focus of light needs to be shifted slightly further back in the eye, so that it lands directly on the retina. This is achieved using a concavely shaped lens. A concave lens spreads out incoming light before it enters the eye, allowing it to be focused further onto the retina, and restoring clear vision.

Hyperopia, also known as far-sightedness, is another type of refractive error in which near objects appear blurry while distant objects can be seen more clearly. This occurs when the eye’s overall focusing power is too weak, often due to a shorter eyeball or a flatter cornea. As a result, light entering the eye is not focused in time and would come to a focal point behind the retina rather than directly on it, leading to blurred near vision.

To correct hyperopia, the focus of light needs to be shifted slightly forward so that it lands directly on the retina. This is achieved using a convexly shaped lens. A convex lens converges (brings together) incoming light rays before they enter the eye, increasing the eye’s focusing power and allowing the image to form correctly on the retina, thereby restoring clear vision for near objects.

In astigmatism, the front surface of the eye (the cornea) has an irregular curvature, rather than being perfectly round and spherical, like a ball. When the front of the eye has an uneven shape, it affects the way light is bent as it passes through the eyeball. Instead of light focusing evenly onto the retina (at the back of the eye), light becomes spread out and unfocused. This leads to blurred or distorted vision at both near and far distances.  

Astigmatism is corrected using a cylindrical lens. Astigmatism usually causes a greater amount of blurred vision along a particular direction more so than others, due to the irregular shape of the eye. The shape of a cylindrical lens bends light that is travelling along a specific direction into clear focus, allowing optometrists and eye doctors to correct for blur from astigmatism.

Myopia often develops during early childhood and worsens every few months until late teenage years or early adulthood. Progressive myopia leads to a gradual decline in vision and a similar increase in eye prescription, and poses a significant concern vision-related quality of life and long-term eye health. Therefore, it is just as important to slow down myopia progression, as it is to correct the blurred vision from myopia.

Choosing the right type of lenses early can play an important role in protecting your child’s long-term vision. While nearly all prescription lenses are designed to correct blurred vision in some shape or form, most lenses do not offer any protection against long-term changes in the eye – for example single vision lenses. Prescription lenses designed to treat worsening myopia are a form of ‘myopia control’, and besides vision correction, they include specially designed “treatment zones” that help to encourage healthy eye growth.