Eye Care Blog

The spectacle lens that corrects short sight or myopia is a negative power: light rays from distant objects are bent in such a way that they become more, not less, divergent. The degree of divergence will depend on the power of the lens. When the degree of (artificial) divergence is equal to the degree of short sight, the eye will form a clear image on the retina. A usual degree of short sight is up to —8.00 dioptres. But in fact it is how healthy the back of the eye remains, and how thin the outer coats of the eye become, that determine the future of the shortsighted person. Whilst the spectacle lens corrects the distance vision, it is still the accommodation of the inner lens that changes the power to enable the eye to see near objects clearly. It must be understood that the spectacle lens makes the image small as compared with normal eyesight. Whilst objects seen with the eye corrected by the minus (negative) lens may be clear, therefore, the smallness of the image may be a disadvantage. One way of overcoming this is by the use of contact lenses. Even without contact lenses many shortsighted people prefer to look at near small objects without their distance spectacles. There is no harm in doing this, although very short-sighted people are unable to use both eyes together when relying only upon their natural sight.

The ideal sight for the non-athletic person is about —1.50 dioptres of myopia. The occasional use of spectacles or contact lenses will give excellent distance vision. For studying and other detailed close work no spectacles are required, even into advanced old age. Many young people, however, do not consider it this way, and ask for operations to correct this small degree of myopia. If such operations are carried out, the strong likelihood is that, when the patient reaches middle age, he will regret losing his ability to see near to without spectacles, which he would otherwise have retained.

Spectacles for short sight not only reduce the size of object- images; they also tend to make them brighter, unless the spectacle lens has been manufactured or coated with a tint. Many very short-sighted people prefer, and may even be advised by their practitioner, to use tinted glasses. Furthermore, the abnormal length of the eye, from the cornea to the back of the retina, which is usually the cause of myopia, often means that the jelly between the crystal lens and the retina is not as firmly attached as it should be. If the eye then changes its contour or volume, as it tends to from early middle age onwards, its internal structures (retina and vitreous jelly) are put under great pressure to adapt, which they cannot readily do, to the new shape. The only way they can adapt is by the retina being unduly stretched and the jelly becoming less solid.

The contact lens operates slightly differently from the spectacle lens. It is in contact (hence the name) with the cornea either directly or with a variable thickness of tear film in- between. Between the spectacle lens and the eye, on the other hand, there is a volume of air. Because the contact lens only has air in front of it, light must be bent to correct short sight almost entirely by its front surface. The reason for this is that, where the fitting of the contact lens permits, the eye and tear fluids have almost as much negative refractive power as the artificial lens itself. Therefore, after light has passed through the contact lens, it will not be further bent until it reaches the lens inside the eye. The contact lens becomes, in effect, the new front of the eye, the new cornea. But because of its proximity to the natural optic system, and because of the materials used in its manufacture, the contact lens is necessarily much thinner than a spectacle lens. For short-sighted eyes, the contact lens also has an additional advantage: it tends to correct any non-spherical problems of the cornea, since the power of the cornea has now been negated (by the additionally thick layer of fluids). The image formed on the retina will now be of a size either equal to that of a normal eye, or larger, and of course the correction itself curves with the eye. But although from an optical point of view the contact lens is significantly more efficient than the spectacle lens, there are some problems associated with the use of contact lenses.

The opposite of short sight is long sight. This occurs when the optical power of the whole eye is too weak to bring the image on to the retina, but instead projects it some distance beyond. The eye is simply too short for the focal length of its lens system. A small degree of long-sightedness can be corrected by `accommodation‘ of the inner lens to a higher power, but if more than two dioptres of accommodation is required in each eye there occurs a simultaneous stimulus for the eyes to converge. But if accommodation is thus required to correct long sight and obtain a clear image of distant objects, this synchronized convergence is of no value since it becomes no longer possible to direct both eyes in the correct direction. Therefore, when there is more than two dioptres of long sight, only one eye is held straight, whilst the other drifts into a corner. Such behaviour is known as strabismus or squinting of the eye. This condition is most common when long sight of high degree is present from birth.

Long sight is very often inherited (as is short-sightedness). It may be due to eyes that have failed to develop to a correct length as the rest of the body grows, or it may be due to a weakness in the cornea or crystal lens. A combination of both factors is also a possibility. Someone suffering from long sight, whilst able to focus easily on distant objects, may find the extra effort required for near vision just too much and consequently develop symptoms of eye-strain. As with short sight, substantial long sight can and should be corrected by the use of spectacles or contact lenses, only in this case the power of the artificial lens needs to be positive in order to bring the image forward on to the retina. Long-sighted spectacles increase the power of the individual’s optics.

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Eye Sight, Short- and Long-Sightedness continued

• Short-sightedness and the Environment part 1
• Eye Sight, Short- and Long-Sightedness
• Eyeglasses / Spectacle Wearers, Eye Make-up Tips
• Eye-Strain, might need Vision Test
• Uncomfortable Eye, Computers and Eyestrain
• Short-sightedness and the Environment part 3
• The Appearance of Contact Lenses part 1
• Eye-Strain, might need Vision Test continue...
• Short-sightedness and the Environment part 2
• Are We Over-using Tinted Eyeglasses Lenses?