Refractive Errors
The human eye is a very sophisticated optical device. For it to work properly, the image of what we are looking at has to be accurately focussed on the retina at the back of the eye. A discrepancy between the focus of the optical components and the position of the retina is termed a “refractive error” and results in the need for glasses or contact lenses. The common refractive errors are myopia, hypermetropia and astigmatism. If there is a perfect match between the optics and the dimensions of the eye we see clearly in the distance without the need to focus or wear spectacles – this condition is called ‘emmetropia’.
The other topics considered here are ‘accommodation’, which is the eye’s ability to change its focus, and the impact of the natural loss of this focussing ability that comes with age which we call ‘presbyopia’.
Myopia (nearsightedness)
Myopia occurs if the optical focus lies in front of the retina. Because the image of a near object comes to a focus behind that of a distant object a myopic individual will often be able to see objects up close clearly, but not those in the distance. This is why the condition is commonly known as ‘nearsightedness’ or ‘shortsightedness’.
Myopia is corrected by concave lenses which are thinner in the centre and thicker at the edges. Such lenses diverge light and push the focal point backwards. When they are worn as spectacles they also have the effect of making objects appear smaller and also induce a prismatic effect when one looks through any part of the lens other than the very centre. This prismatic effect, combined with the change in object size, means that things are not positioned where they seem to be, which affects the wearers spatial perception. Such optical effects are not apparent when contact lenses are worn.
Myopia is usually a result of the eye being too long (axial myopia), but it can occur due to changes in the optical properties of the lens inside the eye. During the development of cataract, the centre, or nucleus, of the lens becomes harder which increases its optical strength. This rise in refractive index draws the focal point of the eye closer. Cataract sufferers often mention that their ability to read has improved but they find it hard to see in the distance because of this ‘index myopia’.
When people develop diabetes the high blood sugar levels cause the lens to swell due to osmotic forces resulting from the high level of glucose inside the lens. This swelling increases the curvature of the surface of the lens which brings the focal point closer and blurs the distance vision. A change in vision of this nature is a clue which sometimes leads to the diagnosis of diabetes, particularly in younger people.
Hypermetropia (longsightedness)
Hypermetropia can be considered the effect of the eye being too short in relationship to the optical elements of the eye. The term longsightedness is in common use because hypermetropia has a greater effect on the near vision, but it has impacts both on the distance and near vision, particularly as people age. Hypermetropia is more common in women who tend to have physically smaller eyes than men. Because of the anatomy of these smaller eyes there is a relationship between hypermetropia and angle closure glaucoma, particularly in older people.
Light is brought to a focus behind the retina in hypermetropic eyes. To correct this, a convex lens is needed – one which is thicker in the centre than the edge. Such lenses are similar to magnifying glasses so when they are worn in spectacles they make the image appear larger, and like the glasses for myopia, induce a prismatic effect – but in the opposite sense.
Younger people can use the natural focussing mechanism (or ‘accommodate’) to overcome milder forms of hypermetropia and see clearly even without glasses, but as we become older that focusing ability is lost – that is we become ‘presbyopic’ (see below) – and the hypermetropia becomes more of a problem.
Furthermore, as we focus on near objects the direction of the eyes also changes from being parallel to align (‘converge’) on the near target. It is not possible for us to focus or accommodate, without the visual axes converging. Therefore, if a child is hypermetropic, they may present with an inturning eye – called a ‘convergent squint’. Correcting the hypermetropia with glasses usually corrects squints of this nature.
There is a special example of hypermetropia we can all experience if swimming without goggles. When water is in contact with the cornea it loses most of its refracting power, which is why everything becomes very blurred.
Astigmatism
The term astigmatism is used to describe the focus of an optical system whose surfaces are not equally curved in each meridian. A lens with unequal curvatures is called a ‘toric’ lens and produces a complex focus called a ‘Conoid of Sturm’ rather than a point focus like a magnifying glass.
Two examples of toric surfaces are a slice off the edge of a doughnut and the surfaces of a rugby ball – both have unequal curvatures. Most of us have at least a subtle degree of astigmatism in our corneas, and it is not unusual for the lens inside the eye also to have astigmatism.
Careful optical measurements determine the astigmatic lens to use to overcome a person’s natural astigmatism problem and such optical correction is commonly included in spectacle or contact lenses. Special toric lens implants are available for use during cataract surgery which have astigmatic surfaces. These produce dramatic improvements in the unaided vision for people with astigmatism caused by the shape of their corneas.
Accommodation and Presbyopia
The lens inside the human eye is suspended from a ring of smooth muscle (the ‘ciliary muscle’) by delicate fibrous ligaments called ‘zonules’. When we look at near objects the ciliary muscle contracts which slackens the zonules and allows the lens to change shape. The lens surfaces become more curved and therefore more powerful optically. This process draws our focal point closer to the eye, allowing the near objects to come into focus. We call this process ‘accommodation’. It is what allows a normally sighted child to see objects as close as a few centimetres from their face as well as in the distance.
As we age the lens becomes harder and less able to change shape. A child’s lens is like a soft gel, but an old person’s lens is hard and fibrous in consistency. This process of gradual hardening of the lens starts when we are very young and gets progressively worse as we age. For a normally sighted person this process of ‘presbyopia’ usually causes problems with reading in the mid 40’s. Because a hypermetropic person is already using some of their accommodation to see clearly in the distance, presbyopia becomes a problem earlier for them.