How do corrective lenses work?
It’s often said that our eyes work a bit like a camera lens.
We see with our eyes, right? Actually, no. We see with the brain
Before we look at how a corrective lens placed in front of the eye helps us to see, we need to take a step backwards.
How, exactly, do we see? The image we see is the result of a complex physiological process that may, at first glance, appear entirely automatic. In actual fact, though, underlying the process of seeing are various openings, lenses, membranes and muscles that are intentionally activated in order to generate a nervous impulse that creates the images we see in our heads.
If you ask an eye-care professional how we see, the first thing they will tell you is that we see what our central nervous system is able to see. More to the point, we could say that we see with the brain because it is here that everything gets interpreted. In a certain sense, the eye is a device — an organ — which acts just like a camera. The eye receives a signal — the light as it enters the eye and reaches the retina at the back of the eye — and sends this signal to the brain, which then processes the signal and interprets it as an image.
In a properly functioning eye, the light comes in through the pupil, which dilates with the help of the muscles of the iris, and focuses as it strikes the retina. For people with vision defects, however, this light comes into focus either before it reaches the retina or behind it. In most cases, this happens because of imperfections in the shape of the eye itself, such as the eyeball being too long or too short or the cornea having an irregular curvature.
Concave lenses for near-sightedness and convex lenses for far-sightedness
When the eye is too long, light comes to a focal point before it reaches the retina. This is known as myopia, more commonly called near-sightedness because people with this defect see better up close than they do from farther away. Because the light comes to focus inside the eye, and not on the retina, the signal sent to the brain results in an image that is out of focus.
An eye that is too short, on the other hand, produces the opposite problem, technically known as hypermetropia, or far-sightedness, since people with this disorder see better from farther away. As the light enters the eye, it would come to focus behind the retina, which causes the defect in our vision.
This is why all we need to do is take a simple, if ingenious, piece of glass that can “bend” the light so that it comes into focus on the retina as it should. This can also be in the form of a “contact” lens, so called because it is placed directly in contact with the surface of the eye, on the cornea.
Lenses used to correct near-sightedness have a “concave” shape, meaning that they are thinner at the center and thicker at the edges. For far-sightedness and presbyopia, conversely, the lenses are “convex”, meaning thinner at the edges than at the center. There is also what is known as an “aspheric” lens, which tends to be thinner, lighter and more aesthetically pleasing and is used to correct certain types of spherical aberration.
How do our eyes focus?
Our eyes essentially serve two distinct functions: an optical function of capturing the light and what we could call the “proto-image”; and a sensing function, which receives the light signal and sends it to the brain. Optical components of the eye include two lenses: the cornea, which bends light towards the retina; and the crystalline lens, a flexible, transparent membrane that is able to correct the focus by altering its curvature. This is how a healthy eye is able to focus on objects at various distances.
The core of the sensing function of the eye is the retina, which is a collection of photosensitive nerve cells concentrated in a film on the innermost surface of the eye. The retina transforms the light into an electrical impulse, which is sent immediately to the cerebral cortex by way of the optical nerves. At the center of the retina is the macula, which is the region most sensitive to light.
In a properly functioning eye, light entering through the pupil is focused on the surface of the retina. As we have seen, in front of the pupil there is a transparent lens known as the cornea, which acts much like a camera lens. Being thicker in the center and thinner towards the edges, it bends the light towards the crystalline lens, which then adapts to more accurately focus the light on the retina to generate a clear, focused image.