How do contact lenses work?

The function of a contact lens is to correct faulty vision or specific problems such as keratoconus or a disfigurement caused by a disease or injury.

This may appear to be an obvious question but it is reasonable to ask as there is more than one reason why people wear contact lenses.

The main reason is to improve your vision but other reasons include changing the appearance of the eye, e.g. coloured or novelty lenses, and as a form of treatment for an eye condition.

Wearing a contact lens

A contact lens is placed into the eye where it sits on top of the cornea and moulds itself to the shape of your eye. It is held in place via tears and pressure from the eyelids. What you will notice is that your lenses move slightly whenever you blink or put pressure on your eyelid. This is a clever process which allows tears from the eye to wash away any dust or debris which has built up in your eye.

This is the way a contact lens is worn. But how does it correct faulty vision?

Before we discuss that, here is a brief overview of the way the eye works and what goes wrong:

Eyesight

The ability to see is based upon the way the eye processes light which is interpreted by the brain as a series of images. As the eye focuses upon an object, light enters the eye through the lens which is then transmitted to the back of the eye.

The back of the eye is called the retina.

The retina contains two types of cells which are responsible for interpreting this light. These cells are known as photoreceptors and are divided into rods and cones.

The rods and cones respond to this light by producing electrical signals which are then transmitted to the brain. The rods are able to detect black and white tones whereas the cones detect colour, e.g. red, blue and green and detail.

The brain combines all these signals together to form an image or what we know as eyesight.

Refractive errors

But many people have a problem with this process in that their eyeball is too long or too short, or the cornea is too curvy/not curvy enough. This causes problems with looking at objects which are near to or far away.

The classic signs of either of those are blurred vision and an inability to see an object clearly when close up or at a distance. The results of this are long sightedness (hypermetropia) and short sightedness (myopia).

Long sightedness

Long sightedness is defined as a problem with seeing objects clearly when they are close to hand.

Short sightedness

Short sightedness is where objects appear blurred when viewed at a distance.

Astigmatism

Another common condition is astigmatism where the cornea is oblong shaped rather than the normal round shape. This causes blurred vision for both near and far objects.

Presbyopia

Presbyopia is another eye condition which occurs when the eye is unable to focus on objects close to hand due to the ageing process. This means having to wear reading glasses once you reach middle age.

All of these visual problems are known as ‘refractive errors’.

Function of a contact lens

So what a contact lens does is to correct these defects so that light is processed in the same way as someone with normal ‘20/20’vision.

The way it does this is dependant upon the design. For example, a lens designed for someone with short sightedness will be thicker on the outside and thinner inside which allows light to be directed onto the correct part of the retina.

Contact lenses work by redirecting light from an object to the right part of the retina so that it is interpreted correctly. This is then sent to the brain and processed as an image (which is what you see in front of you).