Refractive errors and vision

For an image to be seen, the rays of light entering the eye must be focussed (by the refractive action of both the cornea and lens) exactly on the retina. However, this may not happen.

In nearsightedness, or Myopia, the eyeball is too long, and the image focuses in front of the retina. To correct this, a concave lens in the glasses must be used.

In farsightedness or Hypermetropia, the eyeball is too short, and the rays of light focus behind the retina. To correct this, a convex lens in the glasses must be used.

From age 40 onwards, the lens in the eye becomes progressively stiffer, and less able to change shape, to focus near objects (such as print) on the retina. This is known as Presbyopia. Convex reading lenses are needed to correct this. People with perfect distance vision just need reading glasses. A person with myopia and presbyopia requires bifocals where the top half of the lens corrects distance vision and the bottom the near vision.

A modern alternative to this is varifocals, where the division between the two lenses is not noticeable.

Astigmatism

This condition is due to the cornea being “imperfectly shaped”, and therefore not refracting light correctly, causing blurring of vision at certain angles. This can be corrected by specific lenses.

Aphakia

Is the condition where the lens is absent, because it has been removed, or through an eye injury or because it is congenital (the person is born with it). People with this condition need spectacles with a very convex lens, or the insertion of a replacement lens in the eye (intraocular lens).

The correction of refractive errors

Glasses (spectacles)

Refractive errors are corrected by glasses or spectacles. The lenses used to be made of glass, but more recently plastic is usually used, as it does not break (though it scratches more easily) and the lens can be very thin and light which makes them more user – friendly.

Contact lenses

Contact lenses are favoured by many people. Nowadays, gas-permeable contact lenses allow more oxygen to reach and nourish the cornea than the old – style rigid plastic lenses. Soft, water – absorbing lenses are more comfortable to wear, and some can be replaced weekly, others daily, but the use of contact lenses, especially the soft type, requires scrupulous cleaning, and attention to hygiene, or the risk of serious eye infection, even corneal ulcers (which can permanently affect eyesight), may occur.

Surgery

Surgery to correct refractive errors such as near-sightedness and astigmatism especially, is becoming a more popular option.

A laser beam is used to change the shape and refractive ability of the cornea after measurements are made, and recovery is usually quite rapid. The results are usually very successful.