Normal vision depends on light rays from an object under observation focusing on the fovea, the most sensitive part of the retina. To achieve this, the rays are “bent” as they pass through the lens system of the eye, so they all converge on the same central point.
The shape of the lens alters automatically through pressure exerted by the ciliary muscle surrounding it. This enables objects viewed from a distance (six metres away or more) to focus on the fovea in a similar manner to closer objects (those nearer than six metres). When using a camera, lens focus adjustments must be made to cater for the distance of objects; with the eye, however, this is an automatic process, called accommodation.
Although the powers of accommodation are usually very effective, they are sometimes insufficient if the shape or size of the lens is abnormal. For this reason, the point of focus, even with maximum accommodation, may be at a point behind the fovea, resulting in a condition called hyperopia (or hyperuietropia or farsightedness) for objects viewed at a distance. The result is an indistinct image of the object viewed.
About 80 per cent of babies are born hyperopic due to the shape of the eye, but as they develop and the shape of the eye alters, they tend to gain normal vision. Many persons, however, remain slightly hyperopic during adulthood.
The patient may complain of eyestrain, perhaps accompanied by pain in the eyes, headaches and nausea if he has to continually make an effort to accommodate in coping with normal activities.
The opposite to this condition is called myopia (or short sightedness), in which the light rays are brought to a point of focus in front of the retina. A blurred picture is also perceived. Thus the far point of the eye, instead of being in excess of six metres, is usually less than this distance, and will vary according to the degree of myopia. As the myopia increases, so the far point of clear vision decreases. Heredity plays a large part in myopia. It usually increases during the teen years and levels off at 25 years. This happens regardless of other factors, such as the ante – of “close” work done, lighting, rest, exercise, or anything else (most of which have been blamed at some time in the past).
The most common symptom is the inability to distinguish objects clearly in the distance (such as the blackboard at school or road signs). Children are often unaware of their disability because they have never been free from it. Others develop a habit of frowning, and partially close their eyes because this often helps sharpen visual acuity. Over long periods. however, this may cause headaches and eyelid irritation. Myopic children often automatically bring their reading material closer to their eyes than normal.
Recognition of refractive errors and having the eyes tested and corrected is the best course. This is usually carried out by an eye specialist, and frequently accompanies a regular eye examination to detect any other pathological abnormality. Optometrists also carry out refractive tests, manufacturing corrective lenses that give satisfactory results.
With the passage of time, further variations tend to occur with the lens shape, so that regular testing for life may be necessary. Changes will be made to the lens system according to specific needs.