Refractive Errors

Emmetropia is the condition of perfect focus- a distant object is clear with no focussing effort required.An emmetrope would not require a spectacle correction to see clearly, though they would still require reading glasses beyond middle age- see below.

Very few people are truly emmetropic- most  people who do not need spectacles do have a refractive error, but it is so slight that they do not have any problems.

Please note that these pictures are not to scale! They do give an idea what  is going on within the optics of the eye, but the object should be much further  away, beyond 6m- remember that the objects are distant.

 

 

 

 

 

 

 

Here the image of a distant object lies on the retina, with no focussing effort required- the emmetrope sees the test chart letters perfectly clearly, and no error is found.

Myopia (or shortsight) is caused by the focal length of the eye being shorter than its physical length, which causes the focus to form in front of the retina  (the light sensitive layer at the back). This means that far objects will be blurred, but near objects will be clear- depending on the degree of myopia, that clear point may only be a few centimetres from the nose, but still, somewhere up close things will be clear. In general, it is the physical length of the eye which is  wrong- the eyeball is too big, so the focus is in the correct place, but the retina is too far back. Myopia is generally rare in young children- little people  have little eyes, which are usually longsighted! If the eye grows too large then this will cause myopia, which usually begins to show as a problem with the  blackboard in secondary school. Myopia can deteriorate until the eye is fully grown, but may continue to worsen throughout life- it is generally accepted that  those who do more close work will be more likely to progress.

 

 

 

 

 

 

Here the image of a distant object lies in front of the retina, so the image is blurred.

Even with the eye’s muscles completely relaxed, the myope cannot move the focus backwards. The only way a myope will see clearly at distance is with a corrective lens which moves the focus back onto the retina.

Hyperopia (or longsight) occurs when the physical length of the eye is too small for the focal length, which means that without any focussing effort, the focus  would lie  behind the retina. Hyperopes can use the eye’s focussing ability (which should be used to adjust to see close objects) to pull the focus forwards, and thus can usually achieve clear far vision, but this means that there is less focussing ability left in reserve for use when looking at near objects. This is  why hyperopia usually presents with symptoms of eyestrain or blurring when reading.

Hyperopia can’t really deteriorate- eyes don’t shrink, but the strength of glasses can increase over the years- as the ability of the eye to focus  wanes (see presbyopia below) then more help may be required. Many longsighted people do not need to start wearing spectacles until middle forties.

 

 

 

 

 

 

 

In hyperopia the focus would lie behind the retina, and the image would be blurred. Young hyperopes are able to adjust the focussing muscle of the eye,  which increases the eye’s strength and pulls the focus forward, so they usually can achieve clear distance vision, but at the risk of eye strain.

Both of these refractive errors – Myopia and Hyperopia are spherical- that is a spherical lens (one where the power is the same across all meridians)  is used to correct it. It also means that there is a clear focus somewhere, just not in the right place.

Regular astigmatism arises from either an uneven curvature of the cornea (the clear part at the front of the eye), or less commonly a tilt in the  crystalline lens of the eye.

Ideally the cornea would be spherical, and like a sphere- a tennis ball say- no matter how you look at it, it will always have the same curvature.  If you squashed a tennis ball, the top gets closer to the bottom, so the sides must become more curved, but looking the other way, from above, the  curves will not have changed very much- if at all. This means that like a rugby ball, the surface will have a steeper curve in one meridian, and  flatter in the other.

Because the cornea is a lens, and because the curvature of the lens is related to its power, two different curves cause two  different powers. Like the curves of a rugby ball, where the maximum and minimum curves are at right angles, so will the powers always be at 90  degrees to each other.

In astigmatism there will not be clear focal point anywhere- that is where the name comes from (stigmata= a point, astigmata=not a point).

In astigmatism there will be two line foci at 90 degrees to each other.

 

 

 

 

 

 

 

In this eye, with simple myopic astigmatism vertical lines focus onto the retina, but horizontal lines focus in front. Notice how the letters only seem blurred in the one meridian- vertical lines look clear, horizontal lines look blurred.  Everything is blurred along the vertical axis, but this doesn’t affect vertical lines as much as others. This refractive error affects vision both at far and near, and the eye is unable to adjust for it. In general astigmatism which lies  along 90 of 180 degrees- vertical or horizontal causes less trouble than those at an oblique axis.

If the axes were oblique, say at 45 or 135 degrees notice in the left image how the both vertical and horizontal lines are now blurred.

Compound astigmatism with another error such as myopia and it is obvious in the right image that the vision has deteriorated significantly.

Generally babies are born with a moderate amount of astigmatism, which can change throughout the first year to 18 months of life, by which time the cornea has virtually reached its adult size (and shape). From this age therefore, in general astigmatism doesn’t change very much.

Astigmatism can occur on its own, but usually is not in itself the problem- the problem is the long or short sight, and the astigmatism is usually  just the fine tuning.

At approximately mid- forties, people begin to notice that when reading they have to push the print further away- they are unable to focus as well  for near as before. This condition is called presbyopia, and is caused because the lens in the eye never stops growing. This is a problem with near  vision only, but again it can occur in any eye with any other refractive error.

Focussing ability has been in decline since approximately 10 to 12 years of age, but most people don’t notice this until it begins to affect  the distance at which they would commonly read a book. There is no cure or prevention for this natural phenomenon- no amount of eye exercises will make a difference, because it is not the focussing muscle in the eye which is failing, but rather the lens becoming larger which causes the problem.

Reading vision can be improved with spectacles, which improve near vision, but make far vision blurry- that is because all the glasses do is make the wearer artificially shortsighted. Because that means that reading glasses need to be put on and off so many times in a day, or be swapped with distance glasses (imagine trying to read a newspaper and keep an eye on the T.V. at the same time), many people elect to wear bifocals or varifocals which incorporate far and near strengths into the one lens.