Defects of vision and their correction
The so-called normal eye can accommodate for clear vision of objects from infinity (the far point) down to about 25 em (the near point). We shall now consider the case of a person with long sight who can see objects at infinity but whose near point is somewhat further than 25 em (Fig. 24.19). True long
sight, as distinct from loss of accommodation due to advancing age, is caused by the eye-ball being too short. Thus if a printed page is held at the normal 25 cm from such an eye it will appear
blurred since, for this distance, the eye lens can form a sharp image only at a point behind the retina. Correction is effected by a converging spectacle lens which reduces the divergence of the rays entering the eye just sufficiently to make them appear as though coming from the eye’s own more distant near point.
In a case of simple short sight the eye-ball is too long so that the effectively parallel rays from a point for such an eye on a very distant object are focused in front of the retina. The actual far point for such an eye may be only a metre or less away (Fig. 24.20). Correction is obtained by a diverging spectacle lens which diverges the rays entering the eye so that they appear to be coming from the eye’s own far point.