Refractive (focusing) errors of the eye explained

Refractive (focusing) errors of the eye explained

Short-sightedness (Myopia)

Short-sightedness (myopia) occurs when the cornea is too steep or the eye is too long, causing the light entering the eye to focus in the front of the retina. The retinal image is therefore blurred.

If you are short-sighted, you will be able to see close objects clearly, but those further away will be blurred.

The condition is relatively common in the UK, affecting approximately one in five adults. It usually develops as an older child or a teenager and stabilises by the time you are twenty.

If you are not sure whether you are short-sighted, take your glasses off and hold them a few inches away from this screen: the print will look smaller through your glasses if you are shortsighted.

Long-sightedness (Hyperopia)

Long-sightedness is a confusing lay-man’s term since long-sightedness can effect people very differently at depending on the degree of long-sightedness and the age of the patient.

Clinically long-sightedness is known as Hyperopia; it is when the cornea is too flat, or the eye is too short for the relaxed focussing power of the eye.

This means that when an eye is relaxed, the light is focused beyond the retina, blurring any image whatsoever.

A younger person with a smaller degree of hyperopia may be able to overcome this defocus with muscular effort which changes the shape of the internal focussing lens and pulls the focussed light onto the retina. They may in this way recover clear vision in the distance.  The older patient or a patient with a higher degree of hyperopia, will not be able to overcome this defocus.

Hyperopia must be clearly distinguished from Presbyopia. The former is a true refractive error that is far easier to treat by laser eye surgery. Presbyopia on the other hand is a natural ageing process within the eye which is more challenging to treat and requires explanation and understanding. See the below for more information on Presbyopia.

Astigmatism

Astigmatism occurs when the eye is a less rounded shape than usual. Often referred to as being shaped more like a rugby ball (toroidal) rather than a football (Spherical). Astigmatism is created when the cornea of the eye has two different degrees of curvature, one steeper and one flatter which makes it impossible for the eye to focus light perfectly onto the retina.

People with astigmatism will often be able to see vertical objects clearly but horizontal ones may be blurred, or vice versa. Astigmatism is a common disorder, which often co-exists with short or long-sightedness.

Presbyopia

As we get older, the lens within the eye starts to lose its ability to change its shape in order to alter its focus from distance vision to close vision. This natural ageing process is called presbyopia and it is unavoidable.

It can come as quite a shock to some patients in their early/mid forties. It can have two important effects on your vision depending on your type of prescription.

Firstly (most patients), will find during middle age that it becomes harder to focus on near objects or to read for long periods of time.  So you will need reading glasses, or, if you already wear glasses, you will need bifocals or varifocals. If you already wear contact lenses you will now need to wear reading glasses on top of your contact lenses.

Secondly, some patients who may already wear reading glasses, who are only slightly longsighted (perhaps without knowing it) and have previously enjoyed good comfortable distance vision, will find it harder to overcome this slight long-sightedness, and mid range (1-2 meters) and distance vision will start to blur as well requiring either two pairs of spectacles or varifocals. For example – you may have managed without glasses in your 20’s, but needed them in your 30’s or 40’s for reading.  In your 40’s onwards you start to notice that your distance vision is failing as well.

You can see if you have presbyopia by checking your optical prescription, if there is a number written in the “Add box” you will most likely have reached your mid-forties and require an extra power (an “addition”) in the form of separate reading specs, bifocals or varifocals etc.