Vision and Refractive Errors

Light is focused or refracted by the cornea, the clear window of the eye, and the lens inside the eye. Your vision is clear if the cornea and lens continue to focus near and far objects precisely on the retina. The retina is the inner layer of the eye that senses light and helps you to see.

Those who are under the age of 40 can accommodate and shift the focus from near to far objects automatically. Usually when someone is over the age of 40, the ability of the eye to accommodate becomes compromised as the crystalline lens inside the eye becomes thicker and less elastic. This condition is known as Presbyopia, and means that reading spectacles are required. In normal vision, otherwise known as emmetropia, the image is focused sharply onto the fovea, the most sensitive part of the central retina that can discriminate fine detail.

This is caused by a mismatch, or refractive error, between the focusing ability of the cornea and lens on the one hand and the length of the eyeball on the other.

The main refracting surface of the eye is the cornea, which gives the eye approximately 44-45 dioptres of light bending or refracting power. It is, in fact, not the cornea itself but the tear film interface at which the rays of light are refracted. Within the eye the lens is the secondary focusing system, and in a relaxed form contributes approximately 21 dioptres of focusing power. This increases with accommodation so that near objects may be brought to focus on the retina. When this process fails with advancing years the condition is known as presbyopia.

Refractive errors are classified as myopia, or short-sightedness; hyperopia, or long-sightedness; and astigmatism. These disorders may be developmental and usually are so, or they can be acquired because of disease or injury. Normally focusing eyes (emmetropia) are approximately 23mm long from the front surface of the cornea to the retina. Their optical system (cornea and lens) focuses parallel rays of light (objects 6 metres away or more) onto the central retina.

Alternatively, the focusing system at the front of the eye, principally the cornea, may bend rays of light too much, causing the focused image to fall short of the retina.

Without their spectacles, short-sighted people can see near objects clearly, but usually objects beyond arm’s length are blurred or ‘out of focus’. Myopia is a very common defect affecting approximately one in five adults. It often commences in early teenage years and stabilises around 20 years of age. Myopia or short-sightedness is sub-classified into two groups. In physiological myopia, the axial length of the eye as well as the cornea and the lens power are within normal limits for the population but are mismatched, so the image focus is in front of the fovea. Pathological myopia is a more serious form of myopia in which the degree of myopia is generally accepted to be more than 6 dioptres with the eye having an axial length of more than 26.5mm. This is a congenital or neo-natal problem – that it to say, it stems from birth or shortly afterwards. The eyeball may be enlarged but its contents do not grow, but stretch, to fit the globe. This progressive problem may become apparent between the ages of 12 and 50.

In hyperopia or farsightedness, contrary to popular belief, all images are blurred. Distance vision is blurred by a degree according to the degree of hyperopia but no part of the image is strictly clear and near objects are difficult to bring into focus. The 1 dioptre hyperope (+1.00D) would not be able to see at all clearly at a near point of 1 metre and a 10 dioptre hyperope would have a near point of 10 metres.

If you wear hyperopic spectacles, then your glasses will magnify if held a few inches away from a printed page. Your prescription will show a plus (+) sign in front of the ‘sph’ box.

It is a physiological condition and may be of a small degree, which has little effect on the vision. The image formed within the eye has two general points of focus between which the image is often reasonably sharp depending on the degree of astigmatism. Both images may fall short of the retina, one may be on the retina, one in front and one behind, giving rise to the terms such as compound hyperopic, compound myopic or mixed astigmatism. The usual cause of astigmatism is to be found in the cornea which is not spherical but is shaped more like the side of a barrel where it is steeper centrally and flatter at the edges. In the general population, most eyes have astigmatism of a very small degree. Corneal astigmatism is due to a different radius of curvature in two meridians at right angles to each other (regular astigmatism) that causes an image to be focused on two different planes.

If you examine your prescription, you will note a second box usually marked ‘cyl’ and this represents any astigmatism you may have in your spectacle correction. The amount of astigmatism may be preceded by either a plus (+) or a minus (-) sign.

The continuous generation of lens fibres causes increasing compression of the older fibres towards the centre of the lens. In the fifth decade of life, the accumulated changes in lens anatomy reach a critical level in relation to near focusing as the lens gets less flexible. In other words, your arms become too short as you start to move reading material further away to see it more clearly! This loss of accommodation (the ability to change the shape of the crystalline lens) is known as presbyopia. Myopic eyes, without their correction, can of course bring close objects at some point although this might be very close. People often confuse presbyopia with short sight when in fact it is a variation of hyperopia i.e. for near vision only.