At birth, the human infant's eyes are far from fully developed. The world is a blur of lights and fuzzy shapes, colour vision is crude and there is little coordinated movement of the eyes.

Within the first year, the retina (the light sensitive layer of tissue lining the back of the eye like the film in a camera) starts to mature to give colour vision and detailed vision or acuity. The muscle behind the iris (the visibly coloured part of the eye) which focuses (or accommodates) the eye also begins to mature allowing sharp sight over an increasing range of distances. This fine-tuning of the focus of the eye is accomplished by the muscle behind the iris (termed the ciliary body), contracting to allow the lens within the eye to alter its shape and hence alter its focus.

The maturing of the brain, in tandem with the eyes, results in development of coordinated movements that allow tracking of moving objects in all directions and alignment of the eyes during focussing on near objects; the result is one, dynamic mental image of the world.

These processes continue throughout the preschool years, so typically by the age of three, a child has a sharp image of the outside world with colour and depth (or stereopsis). Indeed the process continues further all be it more slowly throughout early childhood and may not be complete until about the age of eight.

Not surprisingly this delicate process, which involves coordinated development of eyes and brain, can be disrupted at many points and most of these problems (the commonest being squint which is a misalignment of the two eyes) are screened for and detected in the U.K. by the preschool health checks. Problems present from birth in infants at risk (e.g. pre-maturity) are evaluated in the maternity unit. More subtle defects may not be detected so an eye examination on entry to primary school is always prudent.

By the age of eight, the visual system is mature but problems such as refractive errors (the need for spectacles or contact lenses to give sharp sight) can begin to manifest anytime onwards especially during adolescence. Sometimes small squints are detectable only for the first time in the early school years.

A general rule (with of course many exceptions) is that hypermetropia (long-sightedness, where the focussing power of the eye is too weak) is manifest from three years plus, and myopia (short-sightedness, where the focussing power of the eye is too strong) is manifest from around the onset of puberty.

An eye examination on entry to secondary school is therefore also prudent.

During life the eyes can be influenced by diseases affecting any of the tissues that make up the eye, or by general disease conditions, most notably diabetes that can affect individuals of all ages. Very often early signs of these and other conditions can be detected by a routine careful eye examination. The modern workplace, be it office or factory floor, places great demands on our eyesight so that even minor defects in vision can become troublesome. For these reasons a routine eye examination every two years is worthwhile.

From the mid-forties we all experience a progressive reduction in accommodation (the range of viewing distance with clear vision). This process (termed presbyopia), which is healthy and not a disease, is a nuisance especially for those who have not had to wear glasses or contact lenses up until now. Modern spectacle lenses and to an extent contact lenses can extend the range of sharp vision, but a check of the prescription along with a full eye examination is advisable every two years.

Glaucoma, which is a major cause of sight loss in the developed world can present at any age but most commonly from the fifth decade onwards. This condition is treatable but is only detected in the early stages by careful eye examination. The tendency to develop glaucoma is inherited so first degree relatives of known glaucoma sufferers should have an annual eye examination from no later than the age of forty: The sight test is paid for under the provisions of the General Ophthalmic Services of the N.H.S.

Cataract, which is eminently treatable, can affect individuals of all ages but becomes more prevalent from the seventh decade onwards. Its development is very gradual. The lens within the eye whose shape is controlled by the ciliary body to focus (or accommodate) the eye continues to grow throughout life. In the early stages of cataract formation, the central portion (or nucleus) becomes denser resulting in a change in the focussing power of the eye, which necessitates a change in spectacle prescription often as frequently as yearly or more. In the early stages, the clarity of vision with appropriate spectacle or contact lens correction is unaffected. When the tissue within the lens starts to become opaque, clarity of vision can start to deteriorate. The rate at which this progresses varies from one individual to another, and even varies between each eye in the same individual. The point at which cataract surgery is considered depends on the individual's requirements and other technical considerations, but a general benchmark is the D.V.L.A.'s standard for visual acuity.

Macular degeneration is also a significant cause of sight loss and can affect all ages, but is more of an issue in the seventh and eighth decades onwards. At present only a small proportion of cases can be treated successfully but much research effort is currently being put into this group of conditions.