A little information on the most common refractive errors- these are the ones correctable with spectacles.
Any irregularity of the shape of the cornea, lens or the eye ball itself may also cause blurrring, but these may not be fully correctable with spectacles alone.
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.
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 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.
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.
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.