Vision and Refractive Errors
What is a refractive error?
Light enters the eye through the cornea (the curved clear window of the eye) and is focused on the retina. Ideally, the cornea should be perfectly dome-shaped. When it is out-of-shape, light bends (refracts) at the wrong angle and images are not focused properly. This causes them to appear blurry or distorted. Problems with the focusing power of the eye are called refractive errors.
There are three primary types of refractive errors: myopia, hyperopia, and astigmatism. In people with myopia (nearsightedness), the cornea is too curved, and items far away appear blurry. In those with hyperopia (farsightedness), the cornea is too flat, and items nearby and far away appear blurry. In people with astigmatism, the cornea is curved irregularly and items appear distorted. Spectacles and contact lenses are designed to compensate for visual imperfections – they bend the light before it enters the eye, helping the eye to focus.
The ability of an eye to focus near and far objects without the aid of corrective lenses is described as normal vision. 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.z
Young people, that are under the age of 40, have the ability to accommodate, that is to shift the focus from near to far objects automatically. Usually over the age of 40 (but with some individual variation) 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 is able to discriminate fine detail.
If the image formed by the eye is not brought into sharp focus on the retina, but instead falls in front of it or behind it, this will result in a blurred image without glasses. This is caused by a mis-match, 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 as a result 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.
In myopia, as a general rule, the eyeball is too long for the focusing system of the eye. Or, 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 arms 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. Myopic eyes may extend up to 30mm or 35mm in length. A near sighted or myopic eye has a far point of clear vision, which is very close to the eye depending on the degree of myopia. One dioptre of myopia (-1.00 D) would give an eye a far point of 1 metre, meaning that objects further away than that are blurred. Ten dioptres of myopia (-l0.00 D), would give a far point of just 10cm. 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 mis-matched 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 diopters 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.
If you wear myopic spectacles, then by holding them a few inches away from a printed page, you will note that they make everything smaller. Alternatively, if you have a spectacle prescription from your optometrist (Optician) your myopia will be indicated in the ‘sph’ box of the prescription with a minus (-) sign either above or in front of a number.
In high refractive errors, particularly high myopia, the edge thickness of high minus spectacle lenses is not only cosmetically unsatisfactory but the image quality is reduced by minification and optical aberrations. For example there is a ring scotoma (blank area) emanating from the periphery of the lens and the spectacle frame. Aberrations become more obvious the larger the spectacle frame. The visual freedom that beckons with refractive surgical procedures is very appealing to patients who are not only visually disadvantaged with their refractive correction but have very poor vision without it.
Hyperopia, in contrast to myopia, is a condition wherein the image is focused behind the eye – in effect the eyeball is too short for the optical system either due to a small eyeball, that is usually 21 mm or less, or a cornea that is too flat, that is unable to bend the rays of light sufficiently to focus retina.
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 in particular 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.
Astigmatism refers to a condition of the focusing system of the eye where the principle focus lies in more than one plane.
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 may have 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 human crystalline lens matures throughout life, doubling its weight over a 90-year life span. It also doubles in thickness, becomes increasingly yellow, and inevitably develops cataract at some point. 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.