Color blindness can be assessed by a series of tests, one of which is a confusion chart, shown here. Under controlled lighting, a person with normal color vision sees a teapot in the pattern of dots. A person with color-defective vision sees only a cup.

The eyes are complex and delicate organs. They have their own external and internal muscles to move the eyeball and change the shape of the lens, and they are supplied by several cranial nerves the optic nerve for sight, and the oculomotor, trochlear, and abducens nerves for eye movement. They also make use of several specialized types of tissue, notably in the cornea, lens, and retina.
We rely on good eyesight for most of our waking lives. It is the primary sense for perception of events and is important in almost all physical coordination. For most people, even a slight problem with their sight can cause considerable irritation or distress although in fact it is also true that people adapt to small defects remarkably quickly.

Some babies are born with minor eyesight disorders, such as color blindness (which is hereditary) or a squint (strabismus). The former cannot be cured but the latter can usually be corrected by an operation. Other defects develop through later life, and by about the age of 40 or 50, many adults need to wear corrective lenses for some minor or major problem concerning their eyes.

Normal eyes focus light from a distant source precisely on to the fovea of the retina.
Nearsighted (myopic) eyes cannot focus light precisely from a distant source because the focal point is in front of the retina. They can focus light from a near source, however. The eyeball of a nearsighted person is slightly elongated.
Correction of nearsightedness requires a concave (diverging) lens to be placed between a distant light source and the eye.

Focusing problems

The most common eyesight defects involve a difficulty in focusing on objects sharply. Myopia (nearsightedness) occurs when the eyeball is elongated from front to back. Instead of forming precisely on the fovea of the retina, focal images form slightly in front of it, caua ing distant objects to appear blurred. Near ob jects, however, can be seen distinctly.

Hyperopia (farsightedness) is the opposite problem. In this condition, the eyeball is shorter than normal from front to back. The focal point of images is beyond the retina, so near objects appear blurred, whereas distant objects can be seen clearly.

Presbyopia is an age-related disorder in which the lens of the eye gradually loses some of its elasticity, making focusing more difficult.

Astigmatism is a condition in which the cornea, at the front of the eyeball, is unevenly curved. Images are slightly distorted in particular regions, because some of the incoming rays of light are bent more than others.

All these focusing defects can tire and strain the eyes, because the eyes adjust constantly in an attempt to refocus and correct the problem. Eyestrain and headaches are common symptoms of focusing problems that need treatment.

An ophthalmoscope is used by an optician or ophthalmologist to examine the eyes during an eye test.

Other eyesight problems

Many other difficulties, not related to focusing, can also affect the eyesight. Problems occasionally occur with the retina of the eye, for example, if it becomes partly or completely detached from the underlying tissue, perhaps as a result of an accident. This damage can repair itself in some cases, but in others must be repaired by surgery. Techniques using a laser to fix the retina back in place are used increasingly in this operation.

A cataract is a disorder in which the lens of the eye gradually becomes more and more opaque, eventually causing almost total loss of detailed vision. Surgery can often provide effective treatment of this disorder, especially cryosurgery, in which the lens is frozen so that the cataract can be removed. The disorder may also require the lens to be removed totally, in which case artificial lenses must be worn after the operation to compensate for the loss of the natural lens.
Other conditions that can affect the eyesight include migraine headaches, which often produce blurred, narrowed, or distorted vision; albinism, an inherited condition in which the normal pigment of the eye is missing (often associated with photophobia, an excessive sensitivity to light); glaucoma, when increased pressure within the eyeball causes objects to appear slightly blurred, as if they have halos around them (this can permanently damage the optic nerve if untreated); and color blindness, or color-deficient vision, an inherited disorder affecting roughly 8 per cent of men and 5 per cent of women, which is characterized by an inability to a greater or lesser degree to identify one or more of the primary colors. Some infections, such as conjunctivitis, can also have an adverse effect on eyesight.

Normal eyes focus light from a nearby source onto the fovea by changing the shape of the lens.
Farsighted (hyperopic) eyes can focus light from a distant source but cannot focus light from a near source, because the focal point of the latter is behind the retina. The eyeball of a farsighted person is slightly shortened.
Correction of farsightedness requires a convex (converging) lens to be placed between the nearby light source and the eye.

Testing the sight

Problems such as defective vision, a squint, or color blindness are noticed most often in childhood. Accurate diagnosis of these and other defects usually requires a full eye test with an optician or ophthalmologist, who tests each eye separately to see if it has any focusing problems. The eyes are also examined with an ophthalmoscope for any possible damage to the retina. The patient is asked whether he or she feels any strain, headaches, or pain associated with the eyesight, in normal or unusual light conditions. The ophthalmologist then tries several different sample lenses in front of each eye to see which one produces the sharpest image. From these experiments, most eyesight disorders can be identified, and corrective lenses can be prescribed.

Corrective lenses

Corrective lenses can be worn either as glasses or as contact lenses. Glasses can be more comfortable to wear because the lenses do not come into contact with the eyes; they also carry less risk of infection. In addition, bifocal or trifocal lenses can be fitted in glasses to avoid the need to use different lenses for different activities.

Contact lenses give a more accurate image of objects, however, because they work closer to the natural eye lens than do glasses. They also tend to be preferred by people who find glasses embarrassing or inconvenient. New users have to accustom themselves gradually to wearing contact lenses all day, because at first they feel like any other foreign body in the eye; however, once they are in place, accustomed wearers find them almost unnoticeable.

Many types of contact lenses are made: some are small and hard and have to be taken out before sleeping; others are large and soft and can be left in at night. All types have to be cleaned regularly and scrupulously. The ophthalmologist usually gives advice on the best kind of lenses to suit an individual’s needs, preferences, and life style.