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.