How vision is corrected

How vision is corrected

Reading this section will give you a good overall understanding of how vision problems can be corrected


It is important to get a clear understanding of what is correctable and what isn’t.
Vision correction means focusing a sharp image of whatever one is looking at in the distance , on the retina.

This preceding line is critical and it is important to get a clear understanding of it.

What this means is that a person who has either myopia, hyperopia or astigmatism DOES NOT have the image formed on the retina, but rather in front of the retina, behind the retina or some other variant of. Vision correction means putting the image back where it belongs.

So, in a short sighted person, the image of whatever they are looking at is formed in front of the retina and to correct it means putting it back. An image NEEDS to be on the retina in order for it to be seen well.

The words “IN THE  DISTANCE” are there to highlight the fact that we are talking about the person looking at something in the distance. That is, beyond arms length. Being able to see up close is a whole new set of problems and is  discussed in the section on presbyopia. We are assuming here that you are young enough (<45) to be able to focus at near.

Therefore what is correctable is longsighted, shortsighted or astigmatism.

If your problems is none of the above but instead you are just presbyopic (older than 45 and can’t read anymore) then read the sections on presbyopia and monovision as your problem is different.


There are essential two ways to correct the vision problems mentioned above.

  1. Change the power of the surface of the cornea using a laser technique such as Lasik or some variant
  2. Change the power of the lens in the eye

As a general principle, corneal procedures are generally performed in younger patients and lens procedures in older patients.
As the lens plays a role in focusing for up close, it would not be very wise to remove it in a younger person as you will not be able to focus to read. An older person has already lost their ability to read due to presbyopia, so removing the lens doesn’t really matter as much. Furthermore, as you age, the lens starts to become opaque, which is to say that it becomes a cataract so removing it is helpful for this as well.


In LASIK, a corneal flap is created. Traditionally with a microkeratome (knife) but nowadays with a femtosecond laser.

In PRK, the surface epithelium is removed and a laser applied directly to the corneal stroma. Therefore there is no flap creation and therefore it leaves behind a thicker base.
LASIK has largely replaced PRK because it is much more comfortable with virtually no discomfort whatsoever and visual recovery is a matter of days versus weeks. The end results however are really no different and in fact there has been a resurgence of PRK in recent years in certain individuals.
LASIK would be the procedure of choice for younger people with shortsightedness up to about 9 diopters and longsightedness up to about 3 diopters. Astigmatism can be corrected up to about 4 diopters.

For people whose refractive error is out of the range of LASIK, a lens can instead be implanted within the eye. This lens is called a ‘Phakic Lens Implant’. It sits either on the surface of the iris or behind the iris. It is in a sense, a contact lens within the eye. The Visian implantable Collamer lens (ICL) is an example of one that sits behind the iris and in front of the natural lens.

We offer the Visian ICL for patients out of range of Lasik.
In older patients, usually late forties or beyond, refractive lens exchange is usually a better option. In this procedure, the eye’s natural lens is removed and replaced by an artificial one. This is usually done because firstly accommodation (ability to focus) is largely gone by this age and secondly because the age for cataract is arriving so it avoids a second operation for cataract later on. Also, at an older age, LASIK is less predictable.

This video below is a good overview of the various options.